fix(google): keep streamed citations adjacent to text

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Why this matters

Google can report URL fetch activity before final grounding metadata arrives. Streaming that activity immediately separates citations from their answer paragraph, so clients such as Shiny Chat cannot highlight the provider-grounded claim.

What changes

• Continue streaming answer text without buffering.
• Emit Google citations before deferred search and fetch activity once the final metadata-bearing chunk arrives.
• Cover search-only and early-fetch stream ordering while preserving final-turn ordering and provider-supplied grounded spans.
• Temporarily cap OpenAI below 3.0 and regenerate its client argument types because OpenAI 3 switched to httpx2 while chatlas provider adapters still accept httpx clients.

Verification

• UV_CACHE_DIR=/tmp/chatlas-uv-cache uv run pytest tests/test_provider_google.py -k "grounding or web_search or web_fetch" -q
• UV_CACHE_DIR=/tmp/chatlas-uv-cache uv run pyright
• UV_CACHE_DIR=/tmp/chatlas-uv-cache uv run ruff check chatlas/_provider_google.py tests/test_provider_google.py
• A fresh Python 3.12 dependency sync plus regenerated provider types passes Pyright with OpenAI 2.54.0.
• A live Gemini 2.5 Flash request in Shiny Chat confirmed citations precede late search/fetch activity and the fetch completes successfully.