ENH: add streaming and invalid-encoding tests for AnatomicalOrientation

[blowekamp]

Add 5 GTest cases to itkAnatomicalOrientationGTest.cxx that cover the
four operator<< overloads and the default branch of
ConvertStringEncoding, raising line coverage from 76.6 % to 95.0 %
and function coverage from 77.8 % to 100 %.

Coverage detail

Metric	Before	After
Lines	76.6 % (108/141)	95.0 % (134/141)
Functions	77.8 % (14/18)	100 % (18/18)

New tests:

• StreamingCoordinateEnum — all 7 CoordinateEnum values via operator<<
• StreamingPositiveEnum — operator<<(PositiveEnum)
• StreamingNegativeEnum — operator<<(NegativeEnum)
• StreamingOrientation — operator<<(AnatomicalOrientation) (outputs space-separated term strings)
• InvalidNegativeStringEncoding — GetAsNegativeStringEncoding() on an INVALID orientation exercises the default branch of ConvertStringEncoding

Remaining 7 uncovered lines are dead-code guards (default: itkGenericExceptionMacro unreachable from a 3×3 matrix) and gcov static-init artifacts.

AI assistance

• Tool: GitHub Copilot (Claude Sonnet 4.6)
• Role: identified uncovered lines via gcovr, wrote and validated the five new test cases
• All 9 AnatomicalOrientation tests pass locally against the coverage preset build