fix(dart): implement atomic two-pass component validation in MessageProcessor

[diegolopezrm]

Summary

Ports the atomic two-pass component validation from #2332 (Swift) to the Dart implementation.

MessageProcessor._processUpdateComponents validated and applied components in a single pass. If an update payload contained a valid component followed by an invalid one, the valid component was already committed to SurfaceComponentsModel when the error was raised, leaving the surface in a half-updated state that no message describes and the client has no way to recover.

This mirrors the structure #2332 introduced in Swift:

1. Validation pass — every component in the batch is checked for a required id and, when the component does not already exist, a required component type. Any failure throws and the batch is abandoned without touching surface state.
2. Mutation pass — runs only once the whole batch has passed validation.

Reproduction

Before this change, the following left root committed even though the batch was rejected:

processor.processMessages([
  CreateSurfaceMessage(surfaceId: 's1', catalogId: catalog.id),
]);

// throws A2uiValidationError for the second component...
processor.processMessages([
  UpdateComponentsMessage(surfaceId: 's1', components: [
    {'id': 'root', 'component': 'Text', 'text': 'valid'},
    {'component': 'Text', 'text': 'no id'},
  ]),
]);

// ...but 'root' was already in the surface.
surface.componentsModel.get('root'); // non-null before, null after

Behavioural note

The "cannot create a component without a type" check moves from the mutation site into the validation pass. It keeps the same meaning — the type is required only when the component does not already exist, so updating an existing component without repeating its type still works — but it is now evaluated for the whole batch before anything is applied. A test covers that case explicitly so the relaxation is not lost.

Tests

Five tests added to dart/a2ui_core/test/processor_test.dart:

• a fully valid batch still applies every component;
• a batch rejected for a missing id leaves the surface untouched;
• a batch rejected for a missing component type leaves the surface untouched;
• a rejected batch does not overwrite components applied by an earlier accepted batch;
• updating an existing component without repeating its type keeps working.

The three regression tests fail on main and pass with this change. The other two guard against over-restriction.

dart test in dart/a2ui_core: 74 passing (69 before). dart analyze reports the same two pre-existing unnecessary_library_directive infos as main and nothing new.

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[gemini-code-assist]

Code Review

This pull request introduces a two-pass processing mechanism in MessageProcessor to ensure transactional integrity when updating components. The first pass validates all components in a batch (checking for missing IDs or missing types for new components) before any mutations are applied in the second pass, preventing the surface from entering a half-updated state. Comprehensive unit tests have been added to verify this behavior under various success and failure scenarios. There are no review comments, and I have no feedback to provide.

[diegolopezrm]

@googlebot I signed it!

[diegolopezrm]

@googlebot I signed it!

[diegolopezrm]

@googlebot I signed it!