fix: do not pass base_url to ChatGoogleGenerativeAI

The CLI hardcodes backend_url as https://generativelanguage.googleapis.com/v1
for the Google provider and passes it as base_url to ChatGoogleGenerativeAI.
However, the google-genai SDK manages its own endpoint and API versioning
internally — passing an external base_url causes the SDK to construct
incorrect request paths, resulting in 404 Not Found errors for all Gemini
models including stable ones like gemini-2.5-flash.

Remove the base_url forwarding for Google clients so the SDK uses its
default endpoint logic.

Reproducer:
    ChatGoogleGenerativeAI(model="gemini-2.5-flash",
        base_url="https://generativelanguage.googleapis.com/v1").invoke("hi")
    # → ChatGoogleGenerativeAIError: 404 Not Found

tradingagents/llm_clients/google_client.py

@@ -28,8 +28,10 @@ class GoogleClient(BaseLLMClient):
         self.warn_if_unknown_model()
         llm_kwargs = {"model": self.model}
 
-        if self.base_url:
-            llm_kwargs["base_url"] = self.base_url
+        # base_url is intentionally NOT passed to ChatGoogleGenerativeAI.
+        # The google-genai SDK manages its own endpoint and API versioning internally.
+        # Passing a base_url (e.g. https://generativelanguage.googleapis.com/v1)
+        # causes 404s because the SDK appends its own paths onto the override URL.
 
         for key in ("timeout", "max_retries", "callbacks", "http_client", "http_async_client"):
             if key in self.kwargs: