[llm] Add app name for OpenRouter

[xingyaoww]

Short description of the problem this fixes or functionality that this introduces. This may be used for the CHANGELOG

OpenRouter has a way to tracking LLM usage from different App: https://openrouter.ai/

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Give a summary of what the PR does, explaining any non-trivial design decisions

Following this, this PR adds the env var variable to make it easier for OpenRouter to track OpenHands' LLM usage.

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Link of any specific issues this addresses

[enyst]

Just a thought, I wonder if it's a bit cleaner if we add them in llm_config.py rather than llm.py. Specially since we seem to set them always, regardless if the user is using openrouter, they seem to belong in configurations. It's similar after all, with the cloud providers specific vars, of which we have a few.

[xingyaoww]

@enyst good point! I've been thinking whether we need to handle then directly (e.g., check if openrouter is in the model name). But i think if the provider is not openrouter, litellm will not use these directly, which shouldn't create too much issue?

[enyst]

I agree the overhead is not really an issue.

I just think they belong with other provider-specific stuff like aws_region_name, and those are defined in llm_config.py.

Even if we don't make them "actual" config values, just for code organization, I feel like they belong in that file.

[xingyaoww]

@enyst good point! Having LLMConfig as the centralized place for all the stuff is probably a good idea. Just moved it to LLMConfig

[enyst]

I'm thinking to move it completely out of llm.py 🤔 - but let's get this in, so you can focus on the fun edits stuff.