fix(agents): preserve framework failure reasons

[mikemikimike]

Closes #483

Summary

• preserve failed framework-agent task/workflow reasons when the status endpoint omits them
• apply the same fallback to synchronous, asynchronous, and event-streaming framework runs
• add a regression test through the public runtime path

Verification

• .venv\\Scripts\\python.exe -m pytest tests/unit/ai/test_runtime.py -q
• uv run --with ruff ruff check --select E9 src/conductor/ai/agents/runtime/runtime.py tests/unit/ai/test_runtime.py
• .venv\\Scripts\\python.exe -m compileall -q src/conductor/ai/agents/runtime
• git diff --check