Fix: validate multihash in CIDv0 constructor

[sumanjeet0012]

Closes #63

Description

This PR fixes issue #63 by adding validation to the CIDv0 constructor to ensure the provided multihash uses the sha2-256 algorithm and has a 32-byte digest, as required by the CID specification.

Changes

• Updated CIDv0.__init__ to decode and validate the multihash code (must be 0x12 for sha2-256) and length (must be 32). Raises a ValueError if the multihash is invalid.
• Added tests test_cidv0_rejects_non_sha256 and test_cidv0_rejects_truncated_digest in tests/test_cid.py.
• Fixed existing tests in test_cid.py and test_new_features.py that were improperly creating CIDv0 objects using non-compliant hashes or raw byte strings (which now fail due to the new strict validation).

[seetadev]

@sumanjeet0012 : Excellent work on this PR.

Wish to share that I particularly like that you not only implemented the validation logic in CIDv0.__init__, but also added targeted regression tests covering unsupported hash algorithms and truncated digests, while updating the existing test suite to use specification-compliant inputs. That makes the change both robust and maintainable, and helps prevent future regressions.

It is also great to see that 23 out of 24 CI checks are already passing across multiple Python versions and Windows. The remaining documentation check appears to be an environment/configuration issue (docs extras not found) rather than a problem with the validation logic itself, so it should be straightforward to resolve. Overall, this is a clean, well-scoped contribution that improves standards compliance, input validation, and interoperability across the IPLD ecosystem.

Sharing the logs of 1 CI/CD issue, which can be resolved quickly.

Run python -m tox run -r
.pkg: remove tox env folder /home/runner/work/py-cid/py-cid/.tox/.pkg
docs: failed with extras not found for package py-cid: - (available: dev)
docs: FAIL code 1 (0.20 seconds)
evaluation failed :( (0.74 seconds)
Error: Process completed with exit code 1.

Good job, and thank you for putting together such a thoughtful fix along with comprehensive test coverage.

@acul71: We will do a py-cid release this Wednesday once this PR and others are ready for merge.

CCing @johannamoran and @mishmosh.

[seetadev]

LGTM :)