test: remove gemini_cli references in 6 + delete test_ai_client_list_models.py

Drop stale gemini_cli expectations: PROVIDERS expectations in
test_providers_source_of_truth + test_provider_curation, the
test_list_models_gemini_cli function (deleted the whole file since it
was the only test), the test_discussion_compression_gemini_cli function,
the test_gcli_path_updates_adapter function (its setter was removed),
the ui_gemini_cli_path references in test_mma_tier_usage_reset_fix and
test_rag_integration. These are the t1.9 minor edits.

tests/test_rag_integration.py

@@ -48,7 +48,7 @@ def test_rag_integration(mock_project):
   app.history_trunc_limit = 1000
   app.top_p = 1.0
   app.ui_agent_tools = {}
-  app.ui_gemini_cli_path = "gemini"
+  
   app.current_model = "gemini-1.5-flash"
   app.active_project_path = os.path.join(mock_project, "manual_slop.toml")