[langchain-ai/langchain] Fix issue #38975: fix(core,langchain): preserve _TracerCore copy, prevent default_tools mutation, return LogStream send value

Job description

Fix GitHub issue 38975 in langchain ai/langchain: https://github.com/langchain ai/langchain/issues/38975 Repo: https://github.com/langchain ai/langchain Target: Ship a real upstream code or documentation fix for this issue in langchain ai/langchain. Required deliverables: A real code or documentation change against langchain ai/langchain. An upstream pull request linked to issue 38975. Testing notes describing what you validated. 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 the upstream patch plus the PR.