Fix issue #5383: [Bug]: LLM Cost is added to the metrics twice

[openhands-agent]

This pull request fixes #5383.

The issue has been successfully resolved. The AI made targeted changes to prevent double-counting of LLM completion costs by:

1. Implementing a caching mechanism using _hidden_params to store the cost when it's first calculated
2. Modifying the _completion_cost method to check for cached costs before adding new ones to metrics
3. Ensuring that when _post_completion is called, it uses the cached cost value instead of recalculating and re-adding it

For a human reviewer, I would summarize the PR as:
"This PR fixes a cost double-counting issue in the LLM completion process. Previously, the _completion_cost method was being called twice (once during log preparation and once during _post_completion), causing costs to be added to metrics twice. The fix implements a caching mechanism using _hidden_params to store the cost on first calculation and reuse it in subsequent calls, ensuring each completion's cost is only counted once. Test results confirm the fix is working as intended."

This is a clean, focused fix that addresses the specific issue without introducing new complexity or side effects.

Automatic fix generated by OpenHands 🙌

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:cff7441-nikolaik   --name openhands-app-cff7441   docker.all-hands.dev/all-hands-ai/openhands:cff7441

[enyst]

@openhands-agent This PR has a wrong approach. Instead:

• revert this commit
• read carefully the LLM class and the Metrics class
• move self._post_completion(resp) up, maybe before the logging with log_completions
• where we try to log completions, use the value in the metrics instead of any method call

[github-actions]

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

[openhands-agent]

New OpenHands update

[github-actions]

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

[enyst]

Most comments on this PR have been solved. The last review comment is not solved: return cost from _post_completion to use it directly.

[github-actions]

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

[xingyaoww]

LGTM!