Bracketed (terminal-native) paste bypasses read-only mode and modifies the buffer #2674

Summary

When a buffer is in read-only mode, a terminal-native bracketed paste (e.g. right-click paste, middle-click paste, or Ctrl+Shift+V in most terminals) still inserts text into the buffer. Typing and Ctrl+V are correctly blocked, but the bracketed-paste path is not, so a "read-only" buffer can be silently modified.

Steps to Reproduce

1. Open any file: fresh testfile.txt
2. Enable read-only mode (Command Palette → "Toggle Read-Only Mode"). The status bar shows [RO].
3. Confirm editing is blocked:
   • Type some characters → nothing is inserted, status bar shows "Editing disabled in this buffer". ✅
   • Press Ctrl+V (with something on the clipboard) → nothing is inserted, "Editing disabled in this buffer". ✅
4. Now paste using the terminal's own paste (bracketed paste), e.g. right-click → Paste, middle-click, or Ctrl+Shift+V.

(Reproducible from a script via tmux: tmux set-buffer -b pb "PASTED"; tmux paste-buffer -b pb -p targeting the fresh pane — -p wraps the text in bracketed-paste markers, exactly like a real terminal paste.)

Actual Behavior

The pasted text is inserted into the read-only buffer:

• Buffer content changes (the pasted text appears at the cursor).
• The tab title gains a * (unsaved-changes marker).
• Status bar shows "Pasted".
• The [RO] read-only indicator is still displayed the whole time.

Expected Behavior

Bracketed paste should be blocked in a read-only buffer, exactly like typing and Ctrl+V — no insertion, and the "Editing disabled in this buffer" message.

Environment

• Fresh built from master
• Linux, tmux 3.4 (also applies to any terminal that supports bracketed paste — right-click / middle-click / Ctrl+Shift+V)