Sync OpenAI API docs from upstream Docs

[Copilot]

Upstream Docs changed for the openai service. This updates the monorepo OpenAI documentation to match the current OpenAPI surface and guide content.

• Guide sync

  • Refreshed existing OpenAI guides from upstream Docs content:
    • Chat Completions
    • Embeddings
    • Images Generations
    • Images Edits
    • Responses
    • Tasks
  • Preserved local .md guide filenames.

• New endpoint coverage

  • Added guides for OpenAPI paths previously missing from local docs:
    • /openai/models
    • /v1/audio/speech
    • /v1/audio/transcriptions
    • /v1/realtime

• README endpoint inventory

  • Updated the OpenAI README from 5 to 10 documented endpoints.
  • Replaced stale/broken table links with local guide links.
  • Added Tasks, Models, Audio, and Realtime entries.

Example updated API coverage:

| OpenAI Realtime API | `/v1/realtime` | [OpenAI Realtime API Integration Guide](docs/openai_realtime_api_integration_guide.md) |

• Fixes sync: update from Docs (675bc2b docs: sync from PlatformBackend [automated]) #830