fix(@angular/build): capture and report Rolldown warnings in library builder

[alan-agius4]

Updates bundleEntryPoints to capture Rolldown warning logs via onLog and return them to buildAction so warnings are logged via the builder logger instead of console.warn.

[gemini-code-assist]

Code Review

This pull request updates the Angular library builder pipeline to capture and propagate warnings and errors emitted during the Rolldown bundling process, including adding a test case to verify warning logging. The review feedback suggests preserving the original error objects and stack traces instead of converting them to strings. Specifically, it is recommended to change the type of errors to unknown[] in the bundle outputs, push the raw error objects directly in the catch block, and throw the original error or an AggregateError in the build action to avoid losing critical diagnostic details.