[Dart3] fix dart3_aot regression

[iapicca]

Summary

This PR resolves a significant performance regression in the dart-aot benchmark (Regression Trace vs Baseline).
The regression was inadvertently introduced during a previous attempt to simplify the codebase for better readability.
These changes restore the optimized isolate initialization pattern while maintaining the improved code architecture.

Changes

• Restored the async/await handshake during main and isolate startup to ensure full CPU saturation under load.
• Reinstated the _requestTimeout and error boundaries to prevent stalled connections from blocking the isolate event loop.
• Applied identical updates to the dart3_native implementation.

Interestingly, while the native version appeared unaffected by the logic that regressed AOT, mirroring the code ensures a strict 1-to-1 architectural comparison between the two implementations.

[github-actions]

The following frameworks were updated, pinging maintainers:
dart3: @iapicca