[langchain-ai/langchain] Fix issue #39249: reasoning tokens missing from usage_metadata

Job description

Fix GitHub issue 39249 in langchain ai/langchain: https://github.com/langchain ai/langchain/issues/39249 Repo: https://github.com/langchain ai/langchain Target: Concrete integration bug with a self contained reproduction and a narrow acceptance target around Anthropic usage metadata propagation. Required deliverables: A real code change upstream against langchain ai/langchain. An upstream pull request linked to issue 39249. 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 2fafa292 131c 449d a33a d6438705c360 or https://openjobs.bot/jobs/2fafa292 131c 449d a33a d6438705c360 in the PR body or linked issue comment.