Added the requirement for environment variables to the README

[ryuichi1208]

When I run the sample, I get the following error. Added the requirement for the GEMINI_API_KEY environment variable to the README.

Traceback (most recent call last):
  File "/Users/ryuichi/ghq/github.com/ryuichi1208/homecmd/tmp/stt/client/tools/sort.py", line 4, in <module>
    agent = Agent(
            ^^^^^^
  File "/Users/ryuichi/ghq/github.com/ryuichi1208/homecmd/tmp/stt/client/tools/.venv/lib/python3.12/site-packages/pydantic_ai/agent.py", line 274, in __init__
    self.model = models.infer_model(model)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ryuichi/ghq/github.com/ryuichi1208/homecmd/tmp/stt/client/tools/.venv/lib/python3.12/site-packages/pydantic_ai/models/__init__.py", line 464, in infer_model
    return GeminiModel(model_name, provider=provider)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ryuichi/ghq/github.com/ryuichi1208/homecmd/tmp/stt/client/tools/.venv/lib/python3.12/site-packages/pydantic_ai/models/gemini.py", line 133, in __init__
    provider = infer_provider(provider)
               ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ryuichi/ghq/github.com/ryuichi1208/homecmd/tmp/stt/client/tools/.venv/lib/python3.12/site-packages/pydantic_ai/providers/__init__.py", line 66, in infer_provider
    return GoogleGLAProvider()
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/ryuichi/ghq/github.com/ryuichi1208/homecmd/tmp/stt/client/tools/.venv/lib/python3.12/site-packages/pydantic_ai/providers/google_gla.py", line 37, in __init__
    raise UserError(
pydantic_ai.exceptions.UserError: Set the `GEMINI_API_KEY` environment variable or pass it via `GoogleGLAProvider(api_key=...)`to use the Google GLA provider.

[hyperlint-ai]

PR Change Summary

Added requirement for the GEMINI_API_KEY environment variable to ensure proper functioning of the agent.

• Introduced a check for the GEMINI_API_KEY environment variable in the README example.
• Raised a ValueError if the GEMINI_API_KEY is not set before running the agent.

Modified Files

• README.md

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[samuelcolvin]

happy to have a comment in the readme saying "you need to set XXX", but I don't think we should change the code like this.

[ryuichi1208]

Okay, I'll just comment the code and not modify it.

[Kludex]

I think we need to either:

1. Point the users to this page: https://ai.google.dev/gemini-api/docs/api-key
2. Use an OpenAI model instead (just because it's more popular)