On Windows (PowerShell/Windows Terminal), the queue-based modal used for
destructive slash command confirmations deadlocks because prompt_toolkit's
input channel becomes unresponsive when entered from the process_loop daemon
thread. Keystrokes never reach the key bindings, so response_queue.get()
blocks until the 120-second timeout expires.
Fix: fall back to _prompt_text_input (stdin-based) when:
1. sys.platform == 'win32' — Windows console doesn't support the modal reliably
2. Called from non-main thread — key bindings can't fire from daemon threads
3. self._app is not set — existing behavior for tests/non-interactive
This mirrors the thread-aware guard from _prompt_text_input (PR #23454).
9 new regression tests covering Windows detection, non-main thread fallback,
macOS/Linux modal preservation, and integration with _confirm_destructive_slash.
Fixes#30768
Surgical reapply of PR #30773. Original branch was many months stale (911
files / 146k LOC of unrelated reverts); the substantive ~30 LOC change in
cli.py plus the new test file were reapplied onto current main with the
contributor's authorship preserved via --author.
