fix(tui): graceful exit on Ctrl+C and resolve multiline paste corruption

[n24q02m]

Issue for this PR

Closes #11826
Closes #10975
Closes #12767

Type of change

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

What does this PR do?

1. Graceful Exit: Resolves the zombie process issue when closing the TUI. By adding a SIGHUP handler in src/index.ts and a fallback process.exit(0) after standard renderer and event loop cleanup, the application will forcefully exit if background processes (like file watchers or MCP servers) are blocking a clean exit.
2. Double Ctrl+C: Instead of exiting immediately upon hitting Ctrl+C on an empty prompt (which was frustrating), users now receive a warning toast first. Pressing it again within 2 seconds confirms the exit.
3. Multiline Paste: Fixes UI corruption and data loss that occurred when pasting large text. Previously, the system generated an extmark [Pasted ~X lines] but didn't actually insert the text into the internal buffer. Now, the raw text is directly inserted via input.insertText().

How did you verify your code works?

Pasted >150 lines of text successfully without UI corruption. Pressed Ctrl+C twice and confirmed the terminal exits immediately. Ran kill -HUP on the process and verified that all child LSP processes were also cleaned up instead of being orphaned.

Screenshots / recordings

N/A

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

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

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

#13848 - fix(tui): add signal handlers to prevent orphaned processes on terminal close

This PR appears to address a similar issue with graceful process cleanup and preventing orphaned/zombie processes on terminal close. It may overlap with the Ctrl+C graceful exit handling in PR #14440.

All other search results either returned PR #14440 itself or addressed different concerns (MCP server processes, ANSI color codes, memory leaks, etc.).

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[n24q02m]

Closing this PR -- it has merge conflicts, 2 CI failures (e2e windows + test linux), and a similar PR #13848 was already closed without merge. The effort to fix and rebase is not justified given these signals. The underlying issues (#11826, #10975, #12767) remain open for others to tackle with a fresh approach.