feat(db): replace deferred unique constraints with partial unique indexes for LLM model configs

Replace deferred UniqueConstraints with partial unique indexes for user and group default configurations in the LLM model configs table. This change improves performance and ensures at most one default config per user/group while maintaining data integrity. The migration script has been updated accordingly to create and drop the new indexes.

gns3server/schemas/__init__.py

@@ -35,7 +35,6 @@ from .controller.llm_model_configs import (
     LLMModelConfigCreate,
     LLMModelConfigUpdate,
     LLMModelConfigResponse,
-    LLMModelConfigListResponse,
     LLMModelConfigWithSource,
     LLMModelConfigInheritedResponse
 )