fix(openai): refactor websocket responses request merge/repair path

- Consolidate Responses websocket transcript merging by introducing typed input-item parsing for previous request input, previous response output, and appended input, replacing ad-hoc raw JSON-array string merges.
- Keep deduplication behavior in a single shared pipeline with metadata parsed once (type/id/call_id), then dedupe function-calls and item IDs before reserializing.
- Refactor fallback-turn tool-call repair into a unified prepare path that returns repaired payload + turn context, and only commits request state after successful forwarding to avoid stale per-turn history.

Closes: #4913