Add 'Detect Models' button to custom provider forms that calls the
provider's /models endpoint to discover available models automatically.
- Supports OpenAI-compatible, Anthropic-compatible, and Google
Generative AI providers
- Auto-fills context window and max tokens from provider response
- Filters out embedding, reranking, and non-text models
- Removes empty default model rows after detection
- Added comprehensive backend and frontend test coverage
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