### Description
Currently, when setting tenant default models (e.g., chat, embedding,
rerank), the API only accepts the composite name
(`model_name@model_instance@model_provider`). However, some integrations
and front-end features prefer using the database `model_id` (UUID)
directly.
This PR adds support for `model_id` in default model configuration:
1. **Request Binding**: Added `model_id` (optional field) to the request
body schema in the handler.
2. **Database Lookup**: If `model_id` is supplied, the service queries
the database to resolve the respective provider, instance, and model
names.
3. **Security Validation**: Verified that the provider associated with
the resolved `model_id` belongs to the requesting tenant.
4. **Unit Tests**: Added `TestSetTenantDefaultModels_WithModelID` to
verify DB ID resolution and tenant mapping.
3d7b45bbd7