Summary

• Keep the normalized str cache key, but construct Struct from the original format. Invalid non-ASCII bytes formats then reach format validation and raise struct.error.
• Cover Struct and the six module-level entry points with two invalid bytes formats.
• Add a Library NEWS entry.

Issue: #158172

This targets 3.14 because main uses a different implementation and is unaffected. The same cache call is present on 3.13, so this can be backported.

Validation

• Windows 11, Debug x64: PCbuild\build.bat -c Debug succeeded.
• .\python.bat -m test -v test_struct: 45 tests passed.
• .\PCbuild\amd64\python_d.exe -m test test_struct -R :: passed.
• .\python.bat Tools\patchcheck\patchcheck.py: passed.
• Full .\python.bat -m test -j4: 434 test files passed; 4 failed; 54 were skipped or resource denied. test_launcher and test_os also fail on an unmodified build of 3.14 at 32965ea. test_urllib and test_urllib2 passed after clearing HTTP_PROXY and HTTPS_PROXY for the test process.

AI assistance was used in preparing this PR; I reviewed the final changes.

• Issue: struct: non-ASCII bytes format raises UnicodeEncodeError instead of struct.error on 3.13/3.14 #158172