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Previously, collectGeminiSSE() only returned the last chunk received
from the upstream streaming response when converting to non-streaming.
This caused incomplete responses where only the final text fragment
was returned to clients.

For example, a request asking to "count from 1 to 10" would only
return "\n" (the last chunk) instead of "1\n2\n3\n...\n10\n".

This was especially problematic for JSON structured output where
the opening brace "{" from the first chunk was lost, resulting
in invalid JSON like: colors": ["red", "blue"]}

The fix:
- Collect all text parts from each SSE chunk into a slice
- Merge all collected text parts into the final response
- Reuse the same pattern as handleGeminiStreamToNonStreaming
  in antigravity_gateway_service.go

Fixes: non-streaming responses returning incomplete text
Fixes: structured output (JSON schema) returning invalid JSON