PYTHON-6172 C/pure-Python decoders disagree on short UUID payloads

[blink1073]

PYTHON-6172

Changes in this PR

• Removes the C decoder's 16-byte length requirement for UUID binary subtypes, matching the pure-Python decoder: such payloads decode to Binary under UNSPECIFIED, and raise InvalidBSON only when UUID conversion is requested.
• Adds a regression test backed by a checked-in payload and a changelog entry.

Test Plan

The regression test decodes a checked-in payload and synthetic short payloads of both UUID subtypes, under the default representation and each UUID representation. python -m unittest test.test_bson.TestBSON passes (70 tests).

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s). No followup work.

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[Copilot]

🔵 Needs a closer look

Enforce exact UUID payload length during conversion and cover the overlong case.

0 open findings

What changed in this PR

Fixes inconsistent C and pure-Python decoding of short UUID binary payloads.

Changes:

• Relaxes the C decoder’s unconditional 16-byte check.
• Adds regression tests and changelog documentation.
• Requires exact-length validation during UUID conversion to reject overlong payloads.

File	Summary
test/​test_bson.py	Adds short UUID payload regression coverage.
doc/​changelog.rst	Documents PYTHON-6172.
bson/​_cbsonmodule.c	Aligns short-payload handling, but overlong Java legacy UUID payloads still require validation.

🧠 Review effort: Lite

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[Copilot]

🔵 Needs a closer look

Add coverage for short OLD_UUID_SUBTYPE payloads with UuidRepresentation.STANDARD.

0 open findings

🧠 Review effort: Lite

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[Copilot]

🟡 Changes recommended

Overlong subtype-3 UUID payloads are not yet rejected during conversion.

1 open finding

• Reject overlong JAVA_LEGACY UUID payloads instead of truncating · New

🧠 Review effort: Lite

[Copilot]

🟢 Approval recommended

No unresolved issues block approval.

1 open finding

• Reject overlong JAVA_LEGACY UUID payloads instead of truncating

🧠 Review effort: Lite