feat: add SayGM model provider

[markdavison]

Description

Adds SayGM as an OpenAI-compatible model provider. Closes #13126.

• Dedicated adapter using https://api.saygm.com/v1/
• Nine curated Chat Completions models
• Provider registration, UI metadata, official logo, and documentation
• Sol tool-call request compatibility
• No static prices because routing prices are dynamic

Checklist

• Contributing guide reviewed
• Relevant documentation added
• Existing adapter/build coverage exercised

Screenshot

Provider setup and model-selection screenshots to be attached before marking ready.

Tests

• llm-info and OpenAI-adapters builds pass
• Core and GUI typechecks pass
• Live streaming verification passes

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