feat(ai): add model caching, settings, and commands

- Introduce model caching with persistence to preferences
- Add provider default model and custom settings management
- Implement `/ai switch`, `/ai models`, and improve `/ai start`
- Add model name autocomplete for chat commands
- Update command definitions and help text
- Add unit tests for new functionality

tests/unittests/test_ai_client.h

@@ -40,3 +40,15 @@ void test_ai_providers_find_previous(void** state);
 void test_ai_providers_find_null_search_str(void** state);
 void test_ai_providers_find_empty_search_str(void** state);
 void test_ai_providers_find_case_insensitive(void** state);
+/* Provider default model and settings tests */
+void test_ai_set_provider_default_model(void** state);
+void test_ai_get_provider_default_model(void** state);
+void test_ai_set_provider_setting(void** state);
+void test_ai_get_provider_setting(void** state);
+/* Model caching tests */
+void test_ai_models_are_fresh_initial(void** state);
+/* AI autocomplete integration tests */
+void test_ai_start_provider_autocomplete_only_on_exact(void** state);
+void test_ai_models_find_null_session(void** state);
+void test_ai_models_find_null_provider(void** state);
+void test_ai_providers_find_cycling(void** state);