[3.14] gh-148178: Validate remote debug offset tables on load (GH-148187)

Treat the debug offset tables read from a target process as untrusted input
and validate them before the unwinder uses any reported sizes or offsets.

Add a shared validator in debug_offsets_validation.h and run it once when
_Py_DebugOffsets is loaded and once when AsyncioDebug is loaded. The checks
cover section sizes used for fixed local buffers and every offset that is
later dereferenced against a local buffer or local object view. This keeps
the bounds checks out of the sampling hot path while rejecting malformed
tables up front.
(cherry picked from commit 289fd2c)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>

Author: pablogsal

Misc/NEWS.d/next/Security/2026-04-06-13-55-00.gh-issue-148178.Rs7kLm.rst

@@ -0,0 +1,2 @@
+Hardened :mod:`!_remote_debugging` by validating remote debug offset tables
+before using them to size memory reads or interpret remote layouts.