fix(agent-chat): add safe local runtime preflight

[ucguy4u]

Summary

This PR introduces changes to local model launch preflight in Brain project setup.
Goal: block unsupported or crash-prone local runtime launches before chat opens.

Core outcome:

• local model launch now runs a phased preflight instead of falling straight into first-prompt initialization
• unsupported or memory-unsafe runtimes surface copyable diagnostics and repair actions
• slow setup shows progress and supports cancellation from the project picker

Changes

Code

• Added:
  • phased runtime preparation and diagnostic envelope types in assistant_runtime_service.dart
  • widget coverage for unsupported local runtime launch in model_library_screen_test.dart
• Updated:
  • model_library_screen.dart to run local runtime preflight before selecting a project runtime
  • litert_lm_service.dart with explicit per-model warmup support
  • assistant_runtime_service_test.dart with unsupported, cancelled, and compatibility-blocked preflight coverage

Logic

• Gemini Nano preflight validates support, initializes, and warms before chat launch
• LiteRT local packages validate compatibility and warm before selection completes
• failures no longer trap the user in launch ambiguity; they stay in the picker with diagnostics they can copy into a GitHub issue

API Changes (if any)

• None.

Database Changes (if any)

• None.

Observability / Logging

• Added a user-copyable diagnostic envelope for runtime launch failures.

Performance Impact

• Latency: initial local runtime launch is now explicit and phased
• Throughput: no meaningful change
• Memory/CPU: unchanged runtime cost, but launch failures are blocked earlier

Risks

• cancellation is cooperative between phases, not a hard native abort
• preflight only covers the runtime paths wired through the Brain project picker
• Rollback plan:
  • revert this PR

Testing

• Unit tests:
  • assistant runtime preparation blocked/cancelled paths
• Integration tests:
  • widget coverage for unsupported runtime diagnostics in the project picker
• Manual testing:
  • not run

Deployment Notes

• Config changes:
  • none
• Order of deployment:
  • standard

Related Commits

• fix(agent-chat): add safe local runtime preflight

Notes

• Focus review on preflight gating, diagnostic copy behavior, and local runtime preparation before selection.

Closes #310

[github-actions]

Plugin Module Size Gate

Policy: modules over 3 MB must be delivered as plugins; plugin modules over 5 MB must document cache management.

Module	Size	Type	Status
No changed Dart/Flutter modules detected	n/a	n/a	OK

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

🚀 PR Quick Check Summary

Check	Status	Description
PR Validation	❌ failure	Title format, docs, bundled model guardrail
Code Quality	❌ failure	Analyze, formatting
Core Tests	✅ success	Core package unit tests

💡 Note: Full app tests, coverage reports, and security scans run on merge to main.

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