Issue for this PR

Closes #15592

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Ensure to call session.process.kill() on ptyProcess.onExit to clean up PTY sessions properly.

How did you verify your code works?

(edited) Added an automated test for the issue.

static analysis, and dynamic analysis by hands with `console.log`.

Before the patch:
onExit() -> publish Exited event -> state().delete(id) -> cannot remove() anymore.

After the patch:
onExit() -> publish Exited event -> session.process.kill() -> publish Delete event

Note:
onExit() and remove() are called twice at most, so I have to move state().delete(id) to above and add session.info.status === 'exited' check.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR