zipfile raises UnicodeDecodeError instead of BadZipFile for a member name flagged UTF-8 but not valid UTF-8

Bug report

Bug description:

When a ZIP entry has general purpose bit 11 set (the name and comment are UTF-8), zipfile decodes the member name with .decode('utf-8') without guarding the decode. A member name that is flagged as UTF-8 but holds bytes that are not valid UTF-8 makes the operation raise UnicodeDecodeError instead of zipfile.BadZipFile.

A corrupt archive should surface as BadZipFile, the module's own error type, so that callers that already handle BadZipFile do not also have to catch UnicodeDecodeError. The sibling extra-field decoder _decodeExtra already wraps its own failures in BadZipFile.

Reproduction:

import io, struct, zipfile

name = b'\xff\xfe'          # flagged UTF-8 but not valid UTF-8
flag = 0x800               # general purpose bit 11
lfh = struct.pack(zipfile.structFileHeader, zipfile.stringFileHeader,
                  20, 0, flag, 0, 0, 0, 0, 0, 0, len(name), 0) + name
cd = struct.pack(zipfile.structCentralDir, zipfile.stringCentralDir,
                 20, 0, 20, 0, flag, 0, 0, 0, 0, 0, 0, len(name), 0, 0, 0, 0, 0, 0) + name
eocd = struct.pack(zipfile.structEndArchive, zipfile.stringEndArchive,
                   0, 0, 1, 1, len(cd), len(lfh), 0)
zipfile.ZipFile(io.BytesIO(lfh + cd + eocd))

UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte

The local file header path has the same gap: if the central directory name is valid UTF-8 but the local header name is not, ZipFile.open() / .read() raises UnicodeDecodeError.

Fix: wrap the two UTF-8 filename decodes (_RealGetContents for the central directory and open() for the local file header) so a UnicodeDecodeError is re-raised as BadZipFile, mirroring _decodeExtra.

CPython versions tested on:

3.13, 3.14

Operating systems tested on:

macOS

Linked PRs

• gh-153428

Start in zipfile.py at _RealGetContents and open(), where the issue identifies the two UTF-8 filename decodes. Run the provided in-memory archive reproduction and add or update focused tests so malformed UTF-8 names consistently raise BadZipFile rather than UnicodeDecodeError.