Gemini exposes its OpenAI-compatible endpoint at
https://generativelanguage.googleapis.com/v1beta/openai. Google's own
docs (https://ai.google.dev/gemini-api/docs/openai) list the supported
parameters and inherit OpenAI's 4-entry stop cap. Without an explicit
`stop_max=4` on the registry the default 16 leaks through and the
upstream silently drops the overflow.
Add the backend registry entry, mirror it in the frontend
`PROVIDER_STOP_MAX` map, and pin the cap with a focused unit test.
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