Fix redo after custom undo bindings

[giri256]

Fixes #1703.

Custom key bindings use save_before=True by default, so the key processor saves the current buffer before calling Buffer.undo() or Buffer.redo(). save_to_undo_stack() previously cleared the redo stack immediately. As a result, a second custom undo discarded the state created by the first undo, and a custom redo discarded its own redo state before it could use it.

This defers redo invalidation until the next actual text change. Undo and redo cancel the pending invalidation because they navigate existing history rather than create a new edit. A normal edit after undo still clears redo.

The new tests cover both behaviors:

• repeated undo/redo calls with the default pre-handler snapshots preserve the complete history;
• inserting new text after undo invalidates redo.

Validation:

• pytest tests/test_buffer.py — 11 passed
• ruff check src/prompt_toolkit/buffer.py tests/test_buffer.py
• ruff format --check src/prompt_toolkit/buffer.py tests/test_buffer.py
• mypy src/prompt_toolkit/buffer.py
• full suite — 149 passed, 5 skipped, 4 environment/baseline failures; the same four failures reproduce on untouched upstream (2 Windows console-buffer failures and 2 existing CLI history-input failures)

AI assistance was used during diagnosis and implementation. I reproduced the failure before changing code, reviewed the final two-file diff, verified the edit-invalidates-redo control, and classified the full-suite failures against an untouched upstream worktree.