Fix LLM model tenant authorization

[yaojin3616]

Summary

• scope LLM model list/create requests to the caller's tenant unless the caller is a platform admin
• scope LLM model update and default-selection lookups to the caller's tenant
• add regression coverage for tenant selection and mutation query scoping

Root cause

Organization admins could submit another tenant's model UUID to the update or default-selection endpoints. Those handlers loaded the model by ID without a tenant predicate, allowing changes to the other tenant's Base URL and API key.

Impact

An organization admin can now manage only models in their own tenant. Platform administrators retain cross-tenant management access.

Validation

• PYTHONPATH=. uv run pytest tests/test_llm_model_tenant_scope.py tests/test_llm_tool_capability_probe.py tests/test_model_logical_delete.py
• PYTHONPATH=. uv run ruff check tests/test_llm_model_tenant_scope.py