feat(dart): inject clocks into GenUI timestamps and timeouts (#2239)

[Varun-S10]

Fixes #2239

Description

This PR adds an optional clock and timer injection to dart/a2ui_core.

Previously, action timestamps used hardcoded real-time DateTime.now(), which made tests hard to predict and slow when testing timeouts. With this change, tests can pass a fake clock to control time instantly without sleeping.

What is added:

• Clock & FakeClock (clock.dart): Added helper classes to control time in tests.
• SurfaceModel: Now uses the injected clock for action timestamps instead of hardcoded DateTime.now().
• SurfaceGroupModel & MessageProcessor: Passes the injected clock down to newly created surfaces.
• CancellationSignal.timeout: Allows testing function timeouts using fake timers.
• Backwards Compatibility: All new clock parameters are optional and default to normal system time. Existing code is not affected.
• Unit Tests: Added tests for FakeClock, SurfaceModel, MessageProcessor, and CancellationSignal.

Pre-launch Checklist

One time:

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have built and run at least one sample.

For this PR:

• I have updated the relevant CHANGELOG.md file.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on a fork, I have verified that scripts/e2e_test.sh passes.

If you need help, consider asking for advice on the discussion board.

[gemini-code-assist]

Code Review

This pull request introduces a clock injection feature to enable deterministic testing and timing control across the A2UI system, adding a Clock typedef, a FakeClock class, and integrating clock propagation into SurfaceModel, SurfaceGroupModel, and MessageProcessor. It also adds a CancellationSignal.timeout factory constructor with an injectable timer factory. The reviewer identified a potential synchronization issue in MessageProcessor where an omitted clock parameter could mismatch with an explicitly provided groupModel's clock, and suggested defaulting the clock to groupModel.clock when available.

[gemini-code-assist]

If a custom groupModel is passed to the MessageProcessor constructor but clock is omitted, this.clock will default to systemClock while groupModel might be using a different clock (such as a FakeClock in tests). This can cause dynamically created surfaces to use a different clock than the rest of the group, leading to inconsistent timestamps and hard-to-debug timing issues.

We should default clock to groupModel.clock if groupModel is provided, to ensure they stay in sync.

  MessageProcessor({
    required this.catalogs,
    void Function(A2uiClientAction)? onAction,
    Clock? clock,
    SurfaceGroupModel<T>? groupModel,
  })  : clock = clock ?? groupModel?.clock ?? systemClock,
        groupModel = groupModel ?? SurfaceGroupModel<T>(clock: clock) {

[Varun-S10]

Applied the changes as suggested and added a unit test to verify.