Python: Fix Gemini client support for Gemini API and Vertex AI (#5258)

* Add Gemini and Vertex AI client support

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address Gemini PR review feedback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* removed sample run readme part

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Evan Mattson <35585003+moonbox3@users.noreply.github.com>

python/packages/gemini/samples/gemini_basic.py

@@ -4,9 +4,9 @@
 
 Covers both non-streaming and streaming responses.
 
-Requires the following environment variables to be set:
-- GEMINI_API_KEY
-- GEMINI_MODEL
+Requires ``GOOGLE_MODEL`` or ``GEMINI_MODEL`` and either Gemini Developer API credentials
+(``GEMINI_API_KEY`` or ``GOOGLE_API_KEY``) or Vertex AI settings
+(``GOOGLE_GENAI_USE_VERTEXAI``, ``GOOGLE_CLOUD_PROJECT``, and ``GOOGLE_CLOUD_LOCATION``).
 """
 
 import asyncio
@@ -35,6 +35,7 @@ async def non_streaming_example() -> None:
     """Runs the agent and waits for the complete response before printing it."""
     print("=== Non-streaming ===")
 
+    # 1. Create the agent with the Gemini chat client and local weather tool.
     agent = Agent(
         client=GeminiChatClient(),
         name="WeatherAgent",
@@ -42,6 +43,7 @@ async def non_streaming_example() -> None:
         tools=[get_weather],
     )
 
+    # 2. Ask the agent for a single weather lookup and print the final response.
     query = "What's the weather like in Karlsruhe, Germany?"
     print(f"User: {query}")
     result = await agent.run(query)
@@ -52,6 +54,7 @@ async def streaming_example() -> None:
     """Runs the agent and prints each chunk as it is received."""
     print("=== Streaming ===")
 
+    # 1. Create the same agent configuration for a streaming tool-call example.
     agent = Agent(
         client=GeminiChatClient(),
         name="WeatherAgent",
@@ -59,6 +62,7 @@ async def streaming_example() -> None:
         tools=[get_weather],
     )
 
+    # 2. Ask a multi-location question and stream the model output as it arrives.
     query = "What's the weather like in Portland and in Paris?"
     print(f"User: {query}")
     print("Agent: ", end="", flush=True)
@@ -76,3 +80,14 @@ async def main() -> None:
 
 if __name__ == "__main__":
     asyncio.run(main())
+
+"""
+Sample output:
+=== Non-streaming ===
+User: What's the weather like in Karlsruhe, Germany?
+Result: The weather in Karlsruhe, Germany is currently sunny with a high of 16°C.
+
+=== Streaming ===
+User: What's the weather like in Portland and in Paris?
+Agent: In Portland, it is currently rainy with a high of 11°C. In Paris, it is cloudy with a high of 27°C.
+"""