feat(api): clarify default LLM model config selection logic

- Update API documentation to explicitly describe default configuration selection logic
- Add fallback behavior in repository to use first config when no default is marked
- Clarify difference between `/default` endpoint and `default_config` field
- Document that `default_config` is never null when configs list is not empty

gns3server/db/repositories/llm_model_configs.py

@@ -496,6 +496,10 @@ class LLMModelConfigsRepository(BaseRepository):
                 if config.is_default and default_config is None:
                     default_config = configs_with_source[-1]
 
+        # Fallback: if no config is marked as default, use the first one
+        if default_config is None and configs_with_source:
+            default_config = configs_with_source[0]
+
         return {
             "configs": configs_with_source,
             "default_config": default_config