fix(langchain): handle done markers after stdout without trailing newline

[nu1lx]

Fixes #36696

Summary

• handle shell done markers when stdout does not end with a trailing newline
• preserve stdout content that appears before the marker on the same line
• add regression coverage for both normal and truncated non-newline stdout

Why

ShellSession currently only recognizes completion when a stdout line starts with the done marker. If a command writes stdout without a trailing newline, the injected marker is appended to that same line instead. In that case, the marker is missed and the command can time out even though it already finished.

Testing

• uv run ruff check langchain/agents/middleware/shell_tool.py tests/unit_tests/agents/middleware/implementations/test_shell_tool.py
• uv run pytest tests/unit_tests/agents/middleware/implementations/test_shell_tool.py -k 'stdout_without_trailing_newline or truncated_stdout_without_trailing_newline or stderr_output_labeling'

AI disclaimer

This PR was drafted with the help of an AI coding agent, then reviewed and refined by a human.

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