Fix issue #2947: Feat: make use of litellm's response "usage" data

[enyst]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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

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Link of any specific issues this addresses
Fix #2947

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[enyst]

@openhands-agent Python linting is failing on this PR. You know how to run python linting in this project.

Important: run linting only on the files changed in this PR.

IMPORTANT:
You have access to a GITHUB_TOKEN, and so you can use the github API to get the diff of the PR, then run lint only on the files changed by it.

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[enyst]

We have a problem. In get_token_count method, what type is the messages parameter? Verify three things:

• what type(s) does the litellm token counter function take. You have options how to do that, such as a small script to inspect it.
• what type(s) does our code, that uses get_token_count, send over
• are they both compatible with the type(s) we get in Usage?

Verify and fix accordingly.

[github-actions]

OpenHands started fixing the pr! You can monitor the progress here.

[enyst]

@openhands-agent python linting is failing on this PR. You know how to do python lint in this project. Do it.

[github-actions]

OpenHands started fixing the pr! You can monitor the progress here.

[openhands-agent]

New OpenHands update

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The workflow to fix this issue encountered an error. Please check the workflow logs for more information.

[enyst]

You're doing great, but there is now one unit test that's failing. You know how to run unit tests in your environment. Run them, see what is failing, and fix it. Take into account previous comments and the issue we're fixing in this PR, so you understand the desired behavior of that test.

[github-actions]

OpenHands started fixing the pr! You can monitor the progress here.

[openhands-agent]

New OpenHands update

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The workflow to fix this issue encountered an error. Please check the workflow logs for more information.