fix(legacy): handle cancelled source selection

Summary

• Fixes: When the source selector is cancelled with Ctrl-D, prompt_select returns None and input_handler raises TypeError while evaluating int(None) instead of returning to the main menu.
• Root cause: The next-step input path assumes prompt_select always returns a numeric option and indexes options without handling its documented no-result exit path.

Closes #513

Regression evidence

• Before: uv run python -m pytest tests/legacy/test_pentest_gpt.py -q exited 1

• After: uv run python -m pytest tests/legacy/test_pentest_gpt.py -q exited 0

Verification

• uv run python -m pytest tests/legacy/ -q
• make ci

Scope

• 2 files changed, +27 / -0 lines