Fix children prop bug in the v0.9 Angular ComponentBinder and improve unit tests to use real models

[josemontespg]

Description

This PR refactors the Angular ComponentBinder unit tests for the v0.9 specification to use real, live instances of ComponentContext, ComponentModel, and SurfaceModel from @a2ui/web_core, completely eliminating fragile, verbose, and easily out-of-date mocks.

It also addresses two critical bugs identified in the binder service implementation and adds robust validation coverage.

Motivation

The primary motivation behind this refactoring is to create a highly reliable, realistic safety net. This enables future refactoring of the ComponentBinder implementation with complete confidence, ensuring that its behavior remains completely unchanged and fully compliant with the v0.9 protocol specification.

Key Changes & Added Test Cases

• Real Implementations Integration: Added a clean, reusable test helper to construct real contexts and dynamic state. Assertions are verified directly against the actual, reactive DataModel state rather than call spies.
• Extensive Binding Coverage: Added detailed test cases covering all possible binding styles from the spec file:
  • Basic Literals: strings, numbers, booleans, objects, and nulls.
  • Two-way bindings: validating reactive propagation of updates back up to the DataModel.
  • Single Child references: handling child, trigger, and content with robust fallback logic for falsy values and pre-existing Child structures.
  • Children lists: covering static child ID arrays, dynamic lists, custom Child object arrays, and template-based ChildList expansions.
  • Checks validation: verifying dynamic validation updates and multiple error outputs against the actual state.

Important Bug Fixes Included

1. Loop Variable Leak (Template Leak):
   • Bug: The template variable was declared outside the properties loop. When processing a component with a children list that defined a template, that template would get leaked and incorrectly attached to all subsequent properties.
   • Fix: Moved the template declaration inside the for (const key of Object.keys(props)) loop so it is properly reset to undefined for each property.
   • Coverage: Added a dedicated unit test should not leak template to subsequent properties to prevent regression.
2. Children Property Null Pointer:
   • Bug: Optional children properties resolving to null or undefined would throw a TypeError during componentId resolution in ComponentBinder.bind.
   • Fix: Added safe optional chaining (value?.componentId and value?.path).
   • Coverage: Covered via should handle null/falsy or non-array values by returning an empty array.

Fixes #1477

[gemini-code-assist]

Code Review

This pull request refactors the ComponentBinder test suite by introducing a helper function for context creation and significantly expanding test coverage for property binding, child resolution, and validation logic. In the service itself, optional chaining was added for children property access. Feedback includes a critical bug report regarding a variable leak in the property binding loop, a potential memory leak due to the service's lifecycle management, and a suggestion to return a promise in a test helper to prevent runtime errors.

[andrewkolos]

The change looks good to me, and I verified that the new tests fail before the change 👍 . However, I'm withholding approval per ditman's comment. Feel free to re-request review when addressed

[josemontespg]

This PR fixes the null de-referencing TypeError when is null/undefined (tracked in #1477).

[josemontespg]

Thanks for the review! Ready for another pass

[ditman]

Thanks for finding and fixing this! Let's go!

[ditman]

Before you land this, please rename the PR to address what the bug is fixing, it's more than tests now!