Resolve Gemini grounding redirect citation URLs

feat: surface Gemini search grounding as OpenAI url_citation annotations

feat: raise proxy output cap and defaults to 32768

feat: raise default max_tokens fallback from 4096 to 16384

fix: map thinking blocks to reasoning_content in OpenAI endpoints

Fix gemini-3.1-pro-high by routing to pro-low with thinkingLevel high.