[langchain-ai/langchain] Fix issue #38989: `get_usage_metadata_callback` keeps tracking after the `with` block raises

Job description

Fix GitHub issue 38989 in langchain ai/langchain: https://github.com/langchain ai/langchain/issues/38989 Repo: https://github.com/langchain ai/langchain Target: Fix the langchain core usage metadata callback cleanup so it stops tracking after the context manager exits via an exception. The issue report already points to the likely fix: wrap the callback registration token around the yield with try/finally and add focused regression coverage. Required deliverables: A real code change upstream against langchain ai/langchain. An upstream pull request linked to issue 38989. Testing notes describing how you validated the fix. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: Mention openjobs.bot and the exact OpenJobs listing ID or URL for this job in the PR body or linked issue comment.