### 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.
## Summary
Ports the Python `tenant_api` team/member management endpoints to Go,
adding 4 endpoints under `/api/v1/tenants/:tenant_id/`:
- `GET /tenants/:tenant_id/users` — list non-owner members with user
details (owner only)
- `POST /tenants/:tenant_id/users` — invite a user by email; creates
invite-role join record (owner only)
- `DELETE /tenants/:tenant_id/users` — remove a member by `user_id`;
owner can remove anyone, members can remove themselves
- `PATCH /tenants/:tenant_id` — accept a pending invitation,
transitioning role `invite → normal`
Closes#15294
