Detect mailcap.findmatch pickle payloads

[massy-o]

Summary

• add mailcap.findmatch to the unsafe pickle globals list
• cover the detection with a focused regression test that scans a pickle payload through ModelScan

Why

mailcap.findmatch can execute a matching mailcap entry test command via os.system(). Without this entry, a pickle payload using that callable can scan cleanly even though unpickling can execute shell behavior.

Testing

• uv run --with-editable . --with pytest pytest tests/test_pickle_unsafe_globals.py

[massy-o]

Self-review: this uses the public ModelScan().scan() path instead of importing the pickle scanner directly, so the regression follows the same path users hit from the CLI/API.

[massy-o]

Self-review: this keeps the fix scoped to the specific stdlib gadget observed in the PoC. mailcap.findmatch can execute a mailcap test command through os.system(), so it belongs with the CRITICAL pickle globals.