Normalize the axis in UnitQuaternion.AngVec

[wpyaoxueros]

UnitQuaternion.AngVec(0.5, [1, 2, 3]) currently produces a quaternion with norm greater than one and a matrix that is not the requested rotation. Normalize a nonzero-angle axis before constructing the quaternion, while preserving the documented identity result for zero angle (including a zero axis). A nonzero angle with a zero axis raises ValueError, consistently with the base angle-axis rotation constructor.

Adds regression coverage for norm, angle/axis round-trip, matrix agreement, degrees, unit-axis controls, and zero-angle/zero-axis behavior.

Validation on Python 3.11:

• New regressions against unchanged master: 5 failed, 3 passed; after the fix: 8 passed.
• Full suite: 410 passed, 11 skipped (55 subtests passed).
• Project-pinned Black 23.10.0 and git diff --check passed.

AI assistance: an AI coding agent helped investigate, implement, test, and review this change. No independent human review is claimed.