fix(llm_clients): standardize Google API key to unified api_key param

GoogleClient now accepts the unified `api_key` parameter used by
OpenAI and Anthropic clients, mapping it to the provider-specific
`google_api_key` that ChatGoogleGenerativeAI expects. Legacy
`google_api_key` still works for backward compatibility.

Resolves TODO.md item #2 (inconsistent parameter handling).

tests/test_google_api_key.py

@@ -0,0 +1,39 @@
+import unittest
+from unittest.mock import patch
+
+
+class TestGoogleApiKeyStandardization(unittest.TestCase):
+    """Verify GoogleClient accepts unified api_key parameter."""
+
+    @patch("tradingagents.llm_clients.google_client.NormalizedChatGoogleGenerativeAI")
+    def test_api_key_mapped_to_google_api_key(self, mock_chat):
+        from tradingagents.llm_clients.google_client import GoogleClient
+
+        client = GoogleClient("gemini-2.5-flash", api_key="test-key-123")
+        client.get_llm()
+        call_kwargs = mock_chat.call_args[1]
+        self.assertEqual(call_kwargs["google_api_key"], "test-key-123")
+
+    @patch("tradingagents.llm_clients.google_client.NormalizedChatGoogleGenerativeAI")
+    def test_legacy_google_api_key_still_works(self, mock_chat):
+        from tradingagents.llm_clients.google_client import GoogleClient
+
+        client = GoogleClient("gemini-2.5-flash", google_api_key="legacy-key-456")
+        client.get_llm()
+        call_kwargs = mock_chat.call_args[1]
+        self.assertEqual(call_kwargs["google_api_key"], "legacy-key-456")
+
+    @patch("tradingagents.llm_clients.google_client.NormalizedChatGoogleGenerativeAI")
+    def test_api_key_takes_precedence_over_google_api_key(self, mock_chat):
+        from tradingagents.llm_clients.google_client import GoogleClient
+
+        client = GoogleClient(
+            "gemini-2.5-flash", api_key="unified", google_api_key="legacy"
+        )
+        client.get_llm()
+        call_kwargs = mock_chat.call_args[1]
+        self.assertEqual(call_kwargs["google_api_key"], "unified")
+
+
+if __name__ == "__main__":
+    unittest.main()