Consolidate VRT missing-sources tests and add _helpers/ foundation (epic #2390 PR 1) #2393

PR 1 of the 11-PR epic tracked in #2390. Restructures xrspatial/geotiff/tests/ foundation and folds the VRT missing-sources cluster.

Scope

Foundation:

• New xrspatial/geotiff/tests/_helpers/ with tiff_builders.py, tiff_surgery.py, markers.py.
• Move make_minimal_tiff() out of conftest.py into _helpers/tiff_builders.py.
• Move _tiff_surgery.py into _helpers/tiff_surgery.py.
• Move requires_gpu, requires_loopback, requires_integration markers (and the gpu_available / loopback_available probes) into _helpers/markers.py.
• Re-export from conftest.py so existing imports keep working.

VRT missing-sources consolidation:

• New xrspatial/geotiff/tests/vrt/ with test_missing_sources.py.
• Fold test_vrt_missing_sources_policy_1799.py and test_vrt_missing_sources_policy_2367.py into the new module. The _1799 cases are a strict subset of _2367 (verified by reading both).
• Parametrize over backends and policy variants with descriptive ids (e.g. raise[eager], warn[dask]). No issue numbers in test ids.
• Leave test_vrt_missing_sources_default_raise_1843.py in place. It covers the internal _vrt.read_vrt entry point and the XRSPATIAL_GEOTIFF_STRICT env var path, neither of which is in the consolidated module's surface.

Out of scope:

• Source modules under xrspatial/geotiff/.
• golden_corpus/.
• The pytest_collection_modifyitems socketserver hack in conftest. PR 11 removes it.

Verification

• pytest xrspatial/geotiff/tests/vrt/ -v lists the expected parametrized IDs.
• pytest xrspatial/geotiff/tests/ -x -q passes.
• File count drops by 2; collect count roughly unchanged.
• CLUSTER_AUDIT.md maps every old file::test to its new file::test_id. Deleted before merge.

Part of epic #2390.