Update web_core with Signal improvements and error handling

[gspencergoog]

Description

This PR updates renderers/web_core with Signal-based improvements to the DataContext and core models to support reactive state updates.

Changes

• renderers/web_core/src/rendering/data-context.ts: Refactored DataContext to better leverage Signal patterns.
• renderers/web_core/src/state/*.ts: Updated state models and subscription logic to prevent reactive leaks and align with signals.
• renderers/web_core/src/errors.ts: Unified action/error message processor handling.

Impact & Risks

• No Breaking Changes to existing backward-compatible APIs; sets foundation for v0.9 rendering pipelines.

Testing

1. Run and verify unit tests inside renderers/web_core.

[gemini-code-assist]

Code Review

This pull request introduces significant improvements to the reactive capabilities of web_core, particularly around Signal-based state management and error handling in DataContext. The introduction of the isSignal utility for robust type checking and the new error handling within reactive effects are great steps forward.

I've identified one area for improvement in the new error handling logic: there's a case where non-A2uiExpressionError errors in reactive evaluations could be silently swallowed, making debugging difficult. I've suggested a change to ensure all errors are reported.

Overall, these are solid changes that enhance the stability and predictability of the reactive system. Addressing the feedback will further improve the robustness of the error handling.

[gemini-code-assist]

The current implementation of the catch block only handles A2uiExpressionError. Any other type of error thrown during the reactive evaluation will be silently swallowed. This can make debugging very difficult as there will be no indication of what went wrong, other than the signal's value becoming undefined.

It would be more robust to handle other error types as well, for example by dispatching a generic error to the surface.

} catch (e: any) {
  if (e instanceof A2uiExpressionError) {
    this.surface.dispatchError({
      code: "EXPRESSION_ERROR",
      message: e.message,
      expression: e.expression,
      details: e.details,
    });
  } else {
    this.surface.dispatchError({
      code: "EXPRESSION_ERROR",
      message: e.message ?? `An unexpected error occurred in function ${call.call}.`,
      expression: call.call,
      details: { stack: e.stack },
    });
  }
  // In reactive mode, we should not throw. Instead, reset the signal value.
  resultSig.value = undefined;
}