[tool] Pin platform below 3.2.0 to fix repo-wide analyze failure

[danielleon-cmd]

package:platform 3.2.0 redesigned Platform (and the legacy FakePlatform/LocalPlatform) as final classes. script/tool/test/mocks.dart does class MockPlatform extends Mock implements Platform, which no longer compiles under 3.2.0:

test/mocks.dart:16:44 - The class 'Platform' can't be implemented outside of its library because it's a final class.

script/tool's constraint (platform: ^3.0.2) allows 3.2.0, so this broke on the next pub get picking it up. Since script/tool backs the repo's own "analyze repo tools" postsubmit task, this has failed on every commit to main for about two weeks, surfacing as a persistent tree-status failure unrelated to each individual PR's own changes.

platform 3.1.6 (the version just below 3.2.0) still has the classic abstract class Platform API that MockPlatform relies on, so pinning below 3.2.0 is the minimal fix. Migrating the ~31 test files that use MockPlatform to the new platform API (FakePlatform/TestNativePlatform) is a larger, separate effort better done deliberately rather than as a drive-by fix.

Tracked internally at https://cloudsufi.atlassian.net/browse/FPOCTSMP-35

FPOCTSMP-35

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA. — pending; the CLA bot flagged this as a first-time contribution from this account.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above. — no existing tracking issue covers this specific breakage; happy to file one if a maintainer wants it tracked separately, but didn't want to file one unilaterally as part of this fix.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style (added to the existing NEXT section — script/tool's version bumps are cut separately from individual PRs, as its pre-existing NEXT entries show).
• I updated/added any relevant documentation (doc comments with ///). — not applicable, no public API surface changed.
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹. — this is a dependency constraint fix with no behavior change; the fix is verified by the pre-existing dart analyze/dart test suite itself going from failing to passing (see test plan).
• All existing and new tests are passing.

Test plan

• dart pub get in script/tool — resolves platform to 3.1.6 (was 3.2.0)
• dart analyze in script/tool — no issues found (was 1 error)
• dart test in script/tool — all 1235 tests pass

Note: The Flutter team is currently trialing Gemini Code Assist for GitHub. Comments from gemini-code-assist[bot] aren't authoritative Flutter-team feedback; I'll wait for a human reviewer's guidance on which automated comments (if any) should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩