refactor: Move startSplitBill navigation handling into the UI

[TaduJR]

Explanation of Change

This is part of the ongoing work to move navigation out of our actions and into the UI, so an action only writes data and the screen decides where to send the user.

Before this change, startSplitBill created the split and also navigated to the split chat. This PR takes the navigation out of the action. The two screens that start a scan split (the confirmation step and the scan skip-confirmation) now handle it themselves. Each screen works out the destination chat before calling the action and passes it in, so it navigates to the exact chat the split was created in. When several receipts are split at once, they now share one chat and the user is taken there a single time after the last one.

startSplitBill no longer returns a value either, since only tests were reading it.

Fixed Issues

$ #84631
PROPOSAL: #84631 (comment)

Tests

Test 1: Split one scanned receipt with one person (direct message)

1. Open a one-on-one chat with a person (or start one from + → Start chat).
2. In the message box, tap +, then Split expense.
3. On the Scan tab, add a receipt (photo on mobile, or Choose files / drag-and-drop on web).
4. On Confirm details, tap Split expense.
5. Verify: you land in that one-on-one chat and the split appears there.

Test 2: Split several scanned receipts at once

1. Open a chat (a group is ideal), tap +, then Split expense, then the Scan tab.
2. Add 2 or 3 receipts.
3. On Confirm details, the button should read Create 2 expenses (or 3).
4. Verify:
   • You are taken into the chat only once.
   • All receipts appear in that same one chat, not spread across separate chats.

Test 3: Split a scanned receipt with a group

1. Open an existing group chat, tap +, then Split expense, then the Scan tab, and add a receipt.
2. On Confirm details, tap Split expense.
3. Verify: you land in that group chat with the split shown.

Test 4: Split with a brand-new group of people

1. Tap the green +, then Start chat, select two or more people you don't already share a group with, and create the group.
2. In the new group, tap +, then Split expense, then the Scan tab, and add a receipt.
3. On Confirm details, tap Split expense.
4. Verify: you land in the new group chat (no duplicate chat is created).

Test 5: Quick-action "Split receipt" (skips the details screen)

1. First complete Test 1 or 2 once, so a scanned split becomes your recent action.
2. Tap the green + and look at the Quick action section. It should now show Split receipt.
3. Tap Split receipt. The camera/upload opens directly.
4. Add a receipt.
5. Verify: the split is created without a Confirm details screen, and you land straight in the correct chat.

Test 6: Offline behavior

1. Force the app offline
2. Run Test 1.
3. Verify: you still land in the correct chat and the split appears immediately (may look "pending"/greyed).
4. Open Test tools again (Cmd/Ctrl + D) and turn Force offline off.
5. Verify: the split finishes syncing and stays in the same chat with no duplicate.

• Verify that no errors appear in the JS console

Offline tests

Same as tests

QA Steps

// TODO: These must be filled out, or the issue title must include "[No QA]."
Same as tests

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
  • MacOS: Desktop
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I verified there are no new alerts related to the canBeMissing param for useOnyx
• I followed proper code patterns (see Reviewing the code)
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    • If any non-english text was added/modified, I used JaimeGPT to get English > Spanish translation. I then posted it in #expensify-open-source and it was approved by an internal Expensify engineer. Link to Slack message:
  • I verified all numbers, amounts, dates and phone numbers shown in the product are using the localization methods
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  • I verified proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
  • I verified the JSDocs style guidelines (in STYLE.md) were followed
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  • The file has a description of what it does and/or why is needed at the top of the file if the code is not self explanatory
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  • A similar style doesn't already exist
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  • The assets are optimized and compressed (for SVG files, run npm run compress-svg)
  • The assets load correctly across all supported platforms.
• If the PR modifies code that runs when editing or sending messages, I tested and verified there is no unexpected behavior for all supported markdown - URLs, single line code, code blocks, quotes, headings, bold, strikethrough, and italic.
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• If a new page is added, I verified it's using the ScrollView component to make it scrollable when more elements are added to the page.
• I added unit tests for any new feature or bug fix in this PR to help automatically prevent regressions in this user flow.
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.

Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[melvin-bot]

@linhvovan29546 Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

[codecov]

Codecov Report

❌ Looks like you've decreased code coverage for some files. Please write tests to increase, or at least maintain, the existing level of code coverage. See our documentation here for how to interpret this table.

Files with missing lines	Coverage Δ
src/libs/actions/IOU/Split.ts	76.65% <100.00%> (-0.28%)	⬇️
.../request/step/confirmation/useExpenseSubmission.ts	74.92% <100.00%> (+0.23%)	⬆️
...equestStepScan/components/ScanSkipConfirmation.tsx	78.07% <0.00%> (-0.70%)	⬇️
... and 49 files with indirect coverage changes

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[chatgpt-codex-connector]

Preserve earlier scans when the shared chat write is deferred

When multiple receipts are submitted while a SEARCH or DISMISS_MODAL deferred-write channel is active, every loop iteration reuses this ID before the first startSplitBill optimistic write has executed. Each call therefore concludes that the chat is new and prepares Onyx.METHOD.SET updates for the same report-actions key; when the deferred callbacks run, a later scan replaces the earlier scan's created and IOU actions, leaving the first split invisible while offline and also queueing multiple create-chat requests for the same report. The shared chat must be established once before subsequent split payloads are built, or subsequent writes must merge into it rather than independently treating it as new.

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