Fix TUI elements build/test compile errors (CI fix for #12634)

[warp-dev-github-integration]

Description

Fixes the failing Formatting + Clippy (Linux) check on #12634 (zb/tui-backend/03b-tui-elements). The TUI elements slice did not compile under --all-targets --all-features --tests:

• Lib error: collect_transferable_subtree in crates/warpui_core/src/core/app.rs calls view.child_view_ids(self) on a &StoredView, but the StoredView enum (introduced alongside the additive TUI registry) never gained a child_view_ids delegation. Added it, delegating to the GUI view and returning an empty Vec for the tui variant (matching the other GUI-only hook delegations like self_or_child_interacted_with).
• Test errors: Once the lib compiled, the new TUI element test files failed:
  • tui_view_tests.rs defines TuiEmpty: TuiElement but only imported TuiElement, not the TuiConstraint / TuiSize / TuiRect / TuiBuffer types it references. Added the missing imports.
  • child_view_tests.rs, container_tests.rs, and event_handler_tests.rs use Box::new(()) as a trivial no-op leaf element, but () did not implement TuiElement. Added a #[cfg(test)] no-op impl TuiElement for () next to the trait definition.

This PR targets the zb/tui-backend/03b-tui-elements branch so it can be merged into the original PR.

Validation

• cargo clippy -p warpui_core --all-targets --all-features --tests -- -D warnings passes
• cargo fmt -- --check clean
• cargo nextest run -p warpui_core --features tui — 335 passed, 7 skipped

Conversation: https://staging.warp.dev/conversation/77c61f4b-58dc-4c6b-91c8-cb48f0c04d21

Run: https://oz.staging.warp.dev/runs/019ee253-bd9c-7e7f-acf7-14021692c87e

This PR was generated with Oz.