[langchain-ai/langchain] Fix issue #39363: ShellSession incorrectly times out when command output does not end with a newline

Job description

Fix GitHub issue 39363 in langchain ai/langchain: https://github.com/langchain ai/langchain/issues/39363 Repo: https://github.com/langchain ai/langchain Target: Fix ShellSession completion marker parsing so commands that write stdout without a trailing newline are not falsely treated as timeouts, and add focused regression coverage. Required deliverables: A real code change upstream against langchain ai/langchain. An upstream pull request linked to issue 39363. 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.