fix(openai): capture web search options and streaming citation annotations

[Atharva Mhaske (atharvamhaske)]

Summary

• web_search_options and prediction were missing from Chat Completions request metadata. Both are now captured.
• Streaming responses were dropping URL citation annotations. They're now aggregated into the final message alongside content and tool_calls.

Verification

Confirmed against real API calls, not just SDK types, using a funded key.

• openai-go v1.12.0's ChatCompletionNewParams.WebSearchOptions/Prediction fields are real.
• A real streaming call to gpt-5-search-api with web_search_options set shows delta.annotations on the wire. It arrives whole in one chunk alongside finish_reason, not incrementally like content.
• openai-go's typed ChatCompletionChunkChoiceDelta struct, and the Python/Node equivalents, don't declare an annotations field at all. The SDK types lag the real API. This tracer parses raw JSON directly instead of the typed struct, so that lag doesn't block the fix.
• The originally cited gpt-4o-search-preview/gpt-4o-mini-search-preview models are now deprecated. Used gpt-5-search-api for the real test call instead.

Changes

• trace/contrib/openai/chatcompletions.go: added web_search_options/prediction to the metadata field list. postprocessStreamingResults now accumulates delta.annotations across chunks into the final message.
• trace/contrib/openai/chatcompletions_test.go: a unit test on StartSpan for the metadata fields (no network needed, since web_search_options/prediction require specific model support to call live), plus a real VCR-recorded streaming test against gpt-5-search-api confirming annotations survive aggregation.
• examples/internal/openai-v1/main.go: new chat-web-search-streaming example.

Notes

make ci clean locally.

Closes #127

[Atharva Mhaske (atharvamhaske)]

Abhijeet Prasad (@AbhiPrasad) ready for review, just small gap fix