### What problem does this PR solve?
- Replace embedding model `dimension` metadata with `max_dimension`.
- Add optional `dimensions` metadata for models with fixed selectable
output dimensions.
- Include `max_dimension` and `dimensions` in model list responses.
- Validate requested embedding dimensions before calling provider
embedding APIs.
- Forward SiliconFlow embedding dimensions with the correct `dimensions`
request field.
- Add unit coverage for embedding dimension validation rules.