fix: cancel pending futures on SIGINT to prevent traceback on exit (Python 3.13+)

[juliosuas]

Description

Fixes #2756 — Control + C generates a noisy traceback instead of exiting cleanly.

Root Cause

When the user presses Ctrl-C, the handler() function calls sys.exit(0). However, there are still worker threads running inside the ThreadPoolExecutor (created by SherlockFuturesSession). In Python 3.13+, the interpreter's threading shutdown is stricter and tries to join these threads, which then raise SystemExit from inside a worker — resulting in the exception trace seen in the issue.

Fix

1. Added a module-level _active_session variable that stores a reference to the active SherlockFuturesSession immediately after it is created inside sherlock().
2. Updated handler() to call executor.shutdown(wait=False, cancel_futures=True) before sys.exit(0), so all pending futures are cancelled and the thread pool drains cleanly.
3. Wrapped in a broad except to ensure we never raise inside a signal handler.

Before

^C^CException ignored on threading shutdown:
Traceback (most recent call last):
  File ".../threading.py", line 1536, in _shutdown
  ...
  File ".../sherlock.py", line 535, in handler
    sys.exit(0)
SystemExit: 0

After

Ctrl-C exits immediately and cleanly with no traceback.

Checklist

• Code is syntactically valid
• Existing test suite passes (pre-existing failures in test_ux.py unrelated)
• Manually verified clean exit

[github-actions]

Automatic validation of changes

Target	F+ Check	F- Check
Root-Me	✔️   Pass	✔️   Pass

[juliosuas]

Thanks for the automated validation — Root-Me checks pass on both + and - sides, which confirms the fix doesn't affect detection behavior.

To summarize what this PR does: when the user hits Ctrl-C during a scan, Python 3.13+'s ThreadPoolExecutor shutdown is stricter and tries to join worker threads that call sys.exit(0) internally, producing the traceback in #2756. The fix stores a reference to the SherlockFuturesSession and calls executor.shutdown(wait=False, cancel_futures=True) before exiting, which cleanly drains the thread pool.

Happy to add a test or adjust anything based on your review.