Fix missing the model name in the tracer span

[git-hulk]

Before this PR, the model name was always missing from tracer's span because it wasn't set when creating the LLM request:

And it will be fixed after this PR:

[gemini-code-assist[bot]]

Summary of Changes

Hello @git-hulk, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where the LLM model name was not being captured in tracer spans, hindering proper monitoring and debugging. It achieves this by explicitly setting the model name when an LLM request is initialized. Furthermore, the PR enhances the robustness of the system by relocating the model configuration validation to an earlier stage in the request lifecycle, ensuring that a model is configured before proceeding with an LLM call.

Highlights

• Tracer Span Fix: Ensures the LLM model name is correctly included in tracer spans by setting the Model field in model.LLMRequest during its creation, improving observability.
• Model Configuration Validation: Introduces an early check for a configured LLM model within the runOneStep function, preventing nil pointer dereferences and providing a clearer error message if the model is not set.
• Code Refactoring: Refactors the model configuration check from the callLLM function to the runOneStep function, centralizing the validation logic and streamlining the request processing flow.
• Test Updates: Updates existing test cases in llmagent_test.go to include the Model: "mock" field in expected LLMRequest objects, reflecting the new behavior and ensuring test accuracy.

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[git-hulk]

/gemini review again.

[verdverm]

Is this also why the model name is not in the response or the branch name is empty? Are they related?

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[git-hulk]

Is this also why the model name is not in the response or the branch name is empty? Are they related?

I think it's not related to the empty branch in the response.

[git-hulk]

@kdroste-google @baptmont Could you please merge this PR if there's nothing else to be done?