fix(pty): pty session handle leak

[kikuchan]

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?

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR:

• PR fix: resolve memory leaks and zombie processes from missing cleanup handlers #13186: fix: resolve memory leaks and zombie processes from missing cleanup handlers
  • This PR addresses similar concerns around process cleanup and preventing zombie processes, which relates to the PTY session cleanup issue in PR fix(pty): pty session handle leak #15599.

The other results (PRs #12718 and #12877) are older signal handler PRs that address orphaned processes but appear to be different approaches.

PR #13186 is the most relevant as it deals with the same core issue: ensuring proper cleanup of processes to prevent memory leaks and zombie processes.