ensure_bytes on buf for pcodec decode

[slowjazz]

Decoding with PCodec is causing issues in upstream zarr because it's being expected to handle numpy arrays. It seems like the other codecs in this library perform input validation in their respective implementations and generally support numpy arrays as a buffer-like.

This PR:

• calls ensure_bytes on the input type for PCodec.decode()
• updates unit tests. the former check_encode_decode_array_to_bytes seems to be a more restrictive version of check_encode_decode_array.

ref: zarr-developers/zarr-python#3176

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.00%. Comparing base (73a21c4) to head (80d4de4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
numcodecs/tests/test_pcodec.py	0.00%	3 Missing ⚠️
numcodecs/pcodec.py	50.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (20.00%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (89.00%) is below the target coverage (100.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #756      +/-   ##
==========================================
+ Coverage   88.83%   89.00%   +0.16%     
==========================================
  Files          64       64              
  Lines        2580     2574       -6     
==========================================
- Hits         2292     2291       -1     
+ Misses        288      283       -5     

Files with missing lines	Coverage Δ
numcodecs/tests/common.py	100.00% <ø> (+4.02%)	⬆️
numcodecs/pcodec.py	7.40% <50.00%> (-0.14%)	⬇️
numcodecs/tests/test_pcodec.py	12.90% <0.00%> (ø)

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