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_discussion_compression.py

@@ -64,11 +64,7 @@ def test_discussion_compression_deepseek():
         
         assert result == "DeepSeek summary."
 
-def test_discussion_compression_gemini_cli():
-    ai_client.set_provider("gemini_cli", "gemini-1.5-flash")
-    
-    mock_adapter = MagicMock()
-    mock_adapter.send.return_value = {"text": "CLI summary."}
+
     
     with patch("src.ai_client.GeminiCliAdapter", return_value=mock_adapter):
         result = ai_client.run_discussion_compression("CLI history")