fix(openai): ensure Responses usage includes token detail fields

- add shared `EnsureResponsesUsageDetails` helper to patch `usage` objects with:
  - `output_tokens_details.reasoning_tokens = 0`
  - `input_tokens_details.cached_tokens = 0`
  - for both plain JSON and SSE `data:` frames, including multi-line frames
- apply the helper to OpenAI Response format outputs in non-stream and stream paths across executors/plugins so translated payloads consistently include required usage details
- update websocket/completion payload builders to emit default `usage` detail fields for prewarm/finish responses

Closes: #4985