Improve TMUX dependency error message for BashExecutor setup

[malhotra5]

Summary

• Implements a friendly, actionable error message when tmux is not installed or not found in PATH during BashExecutor initialization.
• Prevents a noisy stack trace and clearly explains that tmux is required locally for the CLI to manage shell sessions.
• Adds unit tests to ensure this behavior is covered and remains stable.

What is the change?

• In openhands_cli/agent_chat.py, wrap BashExecutor initialization in a try/except block.
• Detect libtmux.exc.TmuxCommandNotFound (and similar cases) and print a clear guidance message, including installation examples for macOS and popular Linux distros.
• Return gracefully without proceeding to agent startup when tmux is missing.

Why is this change needed?

• Fixes Improve TMUX dependency error message #14 by replacing an unhelpful stack trace with a user-friendly message that explains the requirement and how to install tmux locally.

How is this tested?

• Added tests/tests_agent_chat.py:
  • test_setup_agent_handles_missing_tmux: Mocks BashExecutor to raise TmuxCommandNotFound and asserts the friendly messages are printed and setup_agent returns (None, None, None).
  • test_setup_agent_handles_missing_tmux_by_name: Ensures we also catch cases where the exception type may not be importable but the name/message indicates tmux is missing.
• All tests pass locally: 13 passed.

Related issues

• Fixes Improve TMUX dependency error message #14

Checklist

• Code compiles and is formatted (ruff/black)
• Lint passes (pre-commit)
• Unit tests added and passing
• Does not change public API

Additional notes

• No documentation changes required. The behavior is self-explanatory in the CLI output.

@malhotra5 can click here to continue refining the PR