geotiff tests: consolidate VRT validation cluster

[brendancol]

Closes #2395. Tracks epic #2390 (PR 2 of 11).

Summary

Folds the validator-side VRT cluster into one parametrised module under xrspatial/geotiff/tests/vrt/test_validation.py. The new file covers four matrices: validator-rule rejections, entry-point parity (read_vrt x _internal_read_vrt x open_geotiff), the narrow-except contract (exception class x default/strict mode), and path containment.

Files removed

• xrspatial/geotiff/tests/test_vrt_validation_2321.py
• xrspatial/geotiff/tests/test_vrt_capability_validator_2371.py
• xrspatial/geotiff/tests/test_vrt_unsupported_2370.py
• xrspatial/geotiff/tests/test_vrt_narrow_except_1670.py
• xrspatial/geotiff/tests/test_vrt_path_containment_1671.py

Also removes the stale xrspatial/geotiff/tests/CLUSTER_AUDIT.md that PR 1 (#2394) left behind on main when its final cleanup commit landed after the merge.

find xrspatial/geotiff/tests -name 'test_*.py' | wc -l drops from 356 to 352 (5 deleted, 1 added).

Notes

• Two cases that were already xfail in test_vrt_unsupported_2370.py stay under pytest.mark.xfail(strict=False) in the consolidated file (mixed source CRS, mixed source dtype widening). The validator does not yet deliver those rejections; the wrappers will start passing without an edit here when it does.
• docs/source/reference/release_gate_geotiff.rst had two citations pointing at deleted files; both are repointed at vrt/test_validation.py so test_release_gate_cites_only_existing_test_files stays green.
• xrspatial/geotiff/tests/CLUSTER_AUDIT_PR2.md lands with this commit and will be deleted in a follow-up commit on this branch before merge per the epic contract.
• Tests-only restructure. No changes to xrspatial/geotiff/ source modules.

Test plan

• pytest xrspatial/geotiff/tests/vrt/test_validation.py -v -- 81 passed, 2 xfailed
• pytest xrspatial/geotiff/tests/ -x -q -- 5709 passed, 68 skipped, 6 xfailed, 1 xpassed
• CI green across numpy / cupy / dask+numpy / dask+cupy

[brendancol]

PR Review: geotiff tests: consolidate VRT validation cluster

Blockers

None.

Suggestions

• xrspatial/geotiff/tests/vrt/test_validation.py:1036-1046 -- _zstd_error_or_skip() is dead code. The helper is never called; the two zstd tests use @pytest.mark.skipif(not _has_zstandard()) and import ZstdError inline. Remove the unused helper.
• xrspatial/geotiff/tests/vrt/test_validation.py:594-609 -- test_warped_subclass_band_rejected_via_open_geotiff swallows the rejection with a four-way pytest.raises((ValueError, NotImplementedError, RuntimeError, UnsupportedGeoTIFFFeatureError)) and never checks the error message. The old test_warped_vrt_subclass_raises from test_vrt_unsupported_2370.py asserted the message named 'warp' or 'vrtwarped' (via _assert_raises_or_xfail keywords). Add a keyword assertion (e.g. assert any(k in str(excinfo.value).lower() for k in ('warp', 'vrtwarped'))) so a future regression that raises a generic ValueError without naming the failure mode actually fails the test.

Nits

• xrspatial/geotiff/tests/vrt/test_validation.py:74-77, 1236-1240 -- _uniq and _unique_dir both build {label}_{uuid.uuid4().hex[:8]} names. _unique_dir could call _uniq and then mkdir, rather than re-rolling the uuid format inline.
• xrspatial/geotiff/tests/vrt/test_validation.py:61-66 -- the alias dance (_package_read_vrt = _public_read_vrt) is one indirection more than needed. Either import once as _package_read_vrt directly, or drop the second name.

What looks good

• The audit in CLUSTER_AUDIT_PR2.md maps every old file::test to its new home and notes the two xfail(strict=False) carryovers from test_vrt_unsupported_2370.py. Row count matches the old files.
• Parametrise IDs are descriptive throughout (reject[bad-resample-alg], truthy[true], entry[package-read_vrt]). No issue numbers leak into test names.
• Both release_gate_geotiff.rst citations are repointed to vrt/test_validation.py so test_release_gate_cites_only_existing_test_files stays green.
• The stale PR-1 CLUSTER_AUDIT.md is removed in the same commit.
• File count drops from 356 to 352 (5 deleted, 1 added), matching the PR description.
• Source-TIFF and VRT filenames carry uuid suffixes so parallel test workers across worktrees do not collide.

Checklist

• Tests-only restructure; no xrspatial/geotiff/ source modules touched
• Entry-point matrix preserved (package read_vrt, internal _vrt.read_vrt, open_geotiff)
• Edge cases (warped / nested / mixed-{crs,dtype,band-count} / mask / resample / path-traversal) preserved
• No premature materialization
• Docstrings on every public test and helper

[brendancol]

Follow-up review after commit 2175775

All four findings from the prior review have been addressed:

Suggestions

• _zstd_error_or_skip() removed (commit 2175775, vrt/test_validation.py).
• test_warped_subclass_band_rejected_via_open_geotiff now asserts the message contains 'warp' or 'vrtwarped' so a generic-ValueError regression fails the test.

Nits

• _unique_dir now calls _uniq(label) instead of re-rolling the uuid format.
• The alias dance is gone; the module imports the backend read_vrt as _package_read_vrt directly.

Verification

• pytest xrspatial/geotiff/tests/vrt/test_validation.py -q: 81 passed, 2 xfailed.
• No source modules under xrspatial/geotiff/ touched.

Only the temporary CLUSTER_AUDIT_PR2.md is left to remove before merge, per the epic contract.