fix(codex): retry empty buffered completions

[Aotricx]

Summary

• detect buffered upstream bodies that end in a successful terminal event (response.completed / response.done) with no semantic output, using the existing reducer (text, thinking, tool, and web-search events all count as semantic)
• retry with previous_response_id dropped so the resend carries full context, bounded at 1 + 10 attempts
• return an explicit 503 (Codex completed without producing output) after exhaustion instead of a successful empty end_turn
• response.incomplete stays non-retryable (a max-tokens truncation would not improve on resend)

Failure mode

The buffered transport path — CCP_CODEX_TRANSPORT=http, and non-streaming client requests generally — translated a terminal-only upstream body into HTTP 200 with "content": [] and stop_reason: "end_turn". Same class of failure as the live-stream path fixed in #70, on the other transport.

TDD evidence

RED (all three smoke tests before the fix):

assertion `left == right` failed: exhausted empty completions must surface an explicit error:
{"content":[],"id":"msg_…","stop_reason":"end_turn",…}
  left: 200
 right: 503
test result: FAILED. 0 passed; 3 failed

GREEN:

smoke_codex_http_retries_empty_completion ... ok
smoke_codex_http_stream_retries_empty_completion ... ok
smoke_codex_http_empty_completions_exhaust_to_service_unavailable ... ok

The detection helper was also grown test-first: six unit tests (terminal-only completed/done → empty; text / tool-call / incomplete-only / no-terminal → not empty) written against a stub and watched fail before implementation.

Relationship to #70

Independent branch from main; complementary to #70 (live WebSocket path). Two hunks are intentionally identical to #70 so a later merge dedupes trivially: the EMPTY_CODEX_COMPLETION_DETAIL 503 mapping branch, and the zero-delay retry test hook in retry.rs (same *_for_tests pattern as the websocket pool/continuation helpers).

Verification

• cargo fmt --all -- --check
• cargo test — 604 passed, 0 failed across unit and integration suites
• exhaustion smoke test asserts exactly 11 upstream attempts, keeping the loop provably bounded

🤖 Generated with Claude Code