fix(tui): fix Ctrl+V paste on Windows/WSL2 (#6560 and #8209)

[TakashiKyoto]

What does this PR do?

Fixes #6560
Complementary to #8209 which fixes image filename extraction from Windows
file copy. This PR focuses on text paste failures and PowerShell escaping issues.

Ctrl+V paste was silently failing on Windows/WSL2 for text containing file
extensions (e.g., image.png, c:\path\file.txt).
Root cause: Bun.file().type infers MIME from extension without checking if file
exists. Text like image.png was treated as an image file path, failed to load,
and silently dropped. Additionally, on WSL2, clipboardy uses Linux clipboard
tools which cannot access Windows clipboard.

Fix:

• Added file.exists() check before treating pasted text as file path
• Added PowerShell clipboard read for Windows/WSL2 (using -EncodedCommand to
  avoid quoting issues with backslash paths)
• Added explicit text paste handler in onKeyDown for terminals without
  bracketed paste support

---

How did you verify your code works?

Manually tested - pasting image paths now works perfectly in windows 11 WSL2 environment.

[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, here are the related PRs I found:

Related PRs:

1. fix(tui): support clipboard image paste with proper filename on WSL2 #8209 - fix(tui): support clipboard image paste with proper filename on WSL2

   • fix(tui): support clipboard image paste with proper filename on WSL2 #8209
   • Why related: This PR is explicitly mentioned in the current PR as complementary. While fix(tui): support clipboard image paste with proper filename on WSL2 #8209 fixes image filename extraction from Windows file copy, PR fix(tui): fix Ctrl+V paste on Windows/WSL2 (#6560 and #8209) #8626 focuses on text paste failures.

2. feat: add input_copy and input_cut keybindings for keyboard-based text selection #7520 - feat: add input_copy and input_cut keybindings for keyboard-based text selection

   • feat: add input_copy and input_cut keybindings for keyboard-based text selection #7520
   • Why related: Also deals with clipboard/paste functionality in the TUI.

3. fix(clipboard): add OSC 52 fallback for SSH/tmux environments #7028 - fix(clipboard): add OSC 52 fallback for SSH/tmux environments

   • fix(clipboard): add OSC 52 fallback for SSH/tmux environments #7028
   • Why related: Addresses clipboard handling issues in terminal environments, though focused on SSH/tmux rather than Windows/WSL2.

The most directly related is #8209, which is already referenced in the current PR description as a complementary fix.