fix(gateway): stream synthesized recovery finals

[BROCCOLO1D]

Summary

• Emits synthesized final text through stream_delta_callback before recovery/guardrail branches close the turn.
• Refs guardrail-halt-style silent stream close at partial_stream_recovery + fallback_prior_turn_content sites #31449.

Why

• partial_stream_recovery and fallback_prior_turn_content assign final_response from local recovery text and then break without another model delta.
• SSE/Open WebUI-style clients can otherwise receive only a finish chunk and render a blank bubble even though Hermes has a final answer.

Changes

• agent/conversation_loop.py: adds a small helper for synthesized final deltas and uses it for guardrail halt, partial stream recovery, and prior-turn fallback finals.
• tests/run_agent/test_run_agent.py: asserts partial-stream recovery and prior-turn fallback text is emitted to stream consumers.

Validation

• python -m ruff check agent/conversation_loop.py tests/run_agent/test_run_agent.py
• python -m pytest tests/run_agent/test_tool_call_guardrail_runtime.py tests/run_agent/test_run_agent.py::TestRunConversation::test_partial_stream_recovery_on_empty_stub tests/run_agent/test_run_agent.py::TestRunConversation::test_prior_turn_fallback_emits_stream_delta -o 'addopts=' -q

Scope

• In scope: final text synthesized by recovery/guardrail paths that would otherwise bypass stream deltas.
• Out of scope: changing retry policy, recovery selection, or the broader SSE protocol.

[alt-glitch]

This is a resubmission of the closed #31475 (identical diff). Competes with #31477 for fixing #31449.

Both PRs address the same gap: recovery-path final text not streamed to SSE/TUI clients. This PR covers 3 sites (guardrail halt + partial_stream_recovery + fallback_prior_turn_content) using a helper function. #31477 covers only the 2 recovery sites but includes a None sentinel flush.

[BROCCOLO1D]

Follow-up pushed in 1140d26 to align the helper with the stream-close behavior called out in the duplicate-scope comparison: _emit_synthesized_final_delta now emits the final text and the None close sentinel in independent guarded calls, so SSE/TUI writers still receive the close sentinel if final text delivery raises. Added regression coverage for the close sentinel and the callback-error path.

Validation:

• python -m pytest tests/run_agent/test_run_agent.py -k 'partial_stream_recovery_on_empty_stub or prior_turn_fallback_emits_stream_delta or synthesized_final_delta_closes_stream_after_callback_error' -o 'addopts=' -q

[BROCCOLO1D]

Closing this as stale: there has been no activity or maintainer follow-up for over a week, and I don't want to leave inactive work open. Happy to reopen/rework if this becomes useful.