Validate extension type codes in Packer.pack_ext_type

[zhechen-cs]

The C extension accepts pack_ext_type(1.5, b"A") and silently writes type code 1. It also accepts negative codes reserved by the MessagePack format, including -1 for timestamps. The pure-Python packer and ExtType already reject these inputs.

Validate the code before writing an extension header, matching the fallback's TypeError for non-integers and ValueError outside 0–127. Tests cover both autoreset modes, unchanged buffered output after a rejected call, subsequent valid packing, and the two valid boundaries.

Validation on macOS arm64, CPython 3.12:

• The new tests fail on the unmodified C extension: 14 failed, 6 passed.
• C extension full suite: 165 passed, 1 skipped.
• Pure-Python full suite: 162 passed, 4 skipped.
• Ruff lint and format checks, and git diff --check, pass.

The existing thread_unsafe marker warning is also present on the unmodified base. Other platforms and free-threaded Python are left to CI.