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

[n24q02m]

Issue for this PR

Closes #11826
Closes #10975

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 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. 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 and confirmed the terminal exits immediately without leaving orphaned processes. Ran bun run typecheck and bun run test.

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]

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