Fix children prop bug in the v0.9 Angular ComponentBinder and improve unit tests to use real models (google#1472)

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ComponentModel, and SurfaceModel from @a2ui/web_core, completely eliminating fragile, verbose, and easily out-of-date mocks.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.

Author: josemontespg

renderers/angular/CHANGELOG.md

@@ -1,5 +1,6 @@
 ## Unreleased
 
+- (v0_9) Fix null de-referencing TypeError in `ComponentBinder` when `children` property is null or undefined. [#1472](https://github.com/google/A2UI/pull/1472)
 - (v0_8) Fix Icon component to handle camelCase and TitleCase names by converting them to snake_case for `g-icon`.
 - (v0_8) Fix Modal component styling and position fixed for overlay.
 - (v0_9) Remove `placeholder` prop support from the `TextField` component, since it was not part of the v0_9 basic catalog schema. [#1372](https://github.com/google/A2UI/pull/1372)