test(genai): add test for ChatGoogleGenerativeAI class transport check (#831)

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## PR Description

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Added a test for PR #822 for parity with the VertexAI transport check
test at
https://github.com/imperorrp/langchain-google/blob/main/libs/vertexai/tests/integration_tests/test_chat_models.py#L788-L803

## Relevant issues

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#791 

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✅ Test

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![image](https://github.com/user-attachments/assets/0b4304b7-1b7c-48e3-9710-fac476500911)

---------

Co-authored-by: Mason Daugherty <[email protected]>

Author: imperorrp

libs/genai/tests/integration_tests/test_chat_models.py

@@ -833,6 +833,25 @@ async def model_astream(context: str) -> list[BaseMessageChunk]:
     assert isinstance(result[0], AIMessageChunk)
 
 
+@pytest.mark.extended
+def test_prediction_client_transport() -> None:
+    model = ChatGoogleGenerativeAI(model=_MODEL)
+    assert model.client.transport.kind == "grpc"
+
+    model = ChatGoogleGenerativeAI(model=_MODEL, transport="rest")
+    assert model.client.transport.kind == "rest"
+
+    async def check_async_client() -> None:
+        model = ChatGoogleGenerativeAI(model=_MODEL)
+        assert model.async_client.transport.kind == "grpc_asyncio"
+
+        # test auto conversion of transport to "grpc_asyncio" from "rest"
+        model = ChatGoogleGenerativeAI(model=_MODEL, transport="rest")
+        assert model.async_client.transport.kind == "grpc_asyncio"
+
+    asyncio.run(check_async_client())
+
+
 def test_search_builtin() -> None:
     llm = ChatGoogleGenerativeAI(model="models/gemini-2.0-flash-001").bind_tools(
         [{"google_search": {}}]