Extract the WS path's inline phase_policy construction into a shared
_build_phase_engine helper so the REST send_message endpoint can reuse
it. Replace the former 501 stub with actual PLAN_EXEC execution:
- REST POST /chat/sessions/{id}/messages with execution_mode=plan_exec
now builds a phase-policy-backed ReActEngine, calls execute()
(non-streaming), and returns a MessageResponse.
- KTD5: PLAN_EXEC bypasses execute_with_fallback_chain — phase policy
and fallback chain are mutually exclusive.
- When plan_exec.enabled=False, REST falls through to the REACT path
(matching WS behavior).
- WS path refactored to call the same helper; behavior unchanged.
Tests:
- Replace TestRestPlanExec501 with TestRestPlanExec (happy path, bad
config → 500, disabled → falls through to REACT, REACT mode unchanged).
- Add TestBuildPhaseEngineHelper covering all return branches:
not-PLAN_EXEC, disabled, empty-config, invalid-config, tool append,
default-policy fallback.
- All 109 tests pass across the three PLAN_EXEC test files.
b032e08866