gh-158200: Harden faulthandler against freed code metadata

[Alishahryar1]

Why

faulthandler.dump_traceback_later() walks other threads without acquiring the GIL so it can diagnose deadlocks. If a target thread releases its current code object during that walk, the watchdog can dereference freed filename, function-name, or line-table metadata and crash instead of producing a diagnostic.

Addresses #158200.

How

Guard code objects and their filename, function-name, line-table, and monitoring metadata before the lock-free traceback path reads them. Preserve readable later fields when one metadata field is invalid, and use ??? only for unavailable fields. Load monitoring pointers with acquire semantics that match their existing release publication, and decode lines from a validated local line-table snapshot.

These checks detect common freed-memory states but do not provide object-lifetime synchronization, so lock-free traversal remains best-effort during concurrent destruction.

• Issue: faulthandler.dump_traceback_later() can segfault while another thread frees code metadata #158200

[python-cla-bot]

All commit authors signed the Contributor License Agreement.