Ensure confirm() returns a boolean after Enter

[zhangli091011]

Summary

Fix prompt_toolkit.shortcuts.confirm() returning an empty string when Enter is pressed without selecting an answer.

Enter previously fell through to PromptSession's normal accept binding, which returned the empty buffer contents. The confirmation session now handles Enter explicitly and continues waiting for y or n, so successful results remain True or False.

The CI environment's current mypy/typeshed versions also exposed an incompatible return annotation in the existing custom PDB trace dispatcher. Its internal return annotation is now Any, matching the range of values accepted by the debugger API without changing runtime behavior.

Fixes #2046.

Tests

• Added regression coverage for Enter followed by y, Y, n, and N
• .venv\Scripts\python.exe -m pytest tests/test_shortcuts.py -q -k confirm
• ruff check src/prompt_toolkit/shortcuts/prompt.py tests/test_shortcuts.py
• ruff format --check src/prompt_toolkit/shortcuts/prompt.py tests/test_shortcuts.py
• Strict mypy check for the changed implementation
• typos .

AI disclosure

OpenCode using myself/gpt-5.6-sol assisted with issue investigation, implementation, and test preparation. I reviewed the change and verification results.