fix(providers/openai): emit source parts for Responses API streaming annotations

[kylecarbs]

Problem

The OpenAI Responses API streaming path was missing a handler for response.output_text.annotation.added events. This meant that url_citation and file_citation annotations—which carry source URLs and titles for web search results—were silently dropped during streaming.

The non-streaming Generate path and the Chat Completions API streaming path both handled annotations correctly; only the Responses API Stream path was affected. This caused downstream consumers (e.g. chatd in coder/coder) to receive tool-call, tool-result, reasoning, and text parts but zero source parts when using OpenAI web search.

Intended upstream target: charmbracelet/fantasy (opened here on fork due to permissions; please merge upstream.)

Fix

Add a case "response.output_text.annotation.added" to the streaming event switch in responses_language_model.go that:

• Parses the annotation from the event (any type in the SDK, deserializes as map[string]interface{})
• For url_citation: yields a StreamPartTypeSource with SourceTypeURL, URL, and title
• For file_citation: yields a StreamPartTypeSource with SourceTypeDocument and the filename as title

This matches the existing behavior in:

• The Responses API Generate path (lines 770-796)
• The Chat Completions API streaming path (parseAnnotationsFromDelta)
• The Anthropic provider streaming path

Test

Updated TestResponsesStream_WebSearchResponse to:

• Include response.output_text.annotation.added events in the mock stream (matching real API behavior observed in recorded test data)
• Assert that two StreamPartTypeSource parts are emitted with correct URL, title, source type, and non-empty IDs