gh-142560: bytearray: prevent UAF in search-like methods by exporting self buffer

[fatelei]

bytearray: prevent UAF in search-like methods by exporting self buffer

Fix a heap use-after-free when bytearray search helpers captured the raw
buffer pointer before normalizing the “sub” argument. A crafted index
or buffer provider could clear/resize the same bytearray during argument
conversion, invalidating the saved pointer and leading to UAF.

Change:
• For bytearray methods find/rfind/index/rindex/count/startswith/endswith/
contains/split/rsplit, export a temporary Py_buffer on self and pass
view.buf/view.len to the Py_bytes* helpers, then release it. While the
export is live, resizing/clearing raises BufferError, preventing stale
pointer dereferences.

Tests:
• Add re-entrancy tests to Lib/test/test_bytes.py that verify BufferError is
raised when index clears the target during find/count/index/rfind/rindex.

This mirrors existing protection used in bytearray.join and removes the
re-entrancy hazard without changing public APIs.

• Issue: Use-after-free in bytearray search methods via re-entrant __index__ #142560