[langchain-ai/langchain] Fix issue #39138: bug(core): `tool_example_to_messages` silently truncates mismatched tool outputs

Job description

Fix GitHub issue 39138 in langchain ai/langchain: https://github.com/langchain ai/langchain/issues/39138 Repo: https://github.com/langchain ai/langchain Target: Implement the concrete code fix described in the GitHub issue with a scoped upstream pull request. Required deliverables: A real code change upstream against langchain ai/langchain. An upstream pull request linked to issue 39138. Testing notes that explain what you verified. 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 actual 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.