Consolidate unit-level tests, slim conftest, close epic #2390 (PR 11) #2414

Final PR of the GeoTIFF test consolidation epic (#2390).

Scope

Three deliverables:

1. Create xrspatial/geotiff/tests/unit/ and fold the pure-unit files in:
   • unit/test_header.py from test_header.py
   • unit/test_geotags.py from test_geotags.py
   • unit/test_safe_xml.py from test_extra_tags_safe_filter_1657.py and test_gdal_metadata_xml_escape_1614.py
   • unit/test_compression.py from the pure-unit codec files (packbits jit, compression docstring/level, etc.)
   • unit/test_dtypes.py for any pure-dtype unit cases
2. Slim conftest.py: drop the pytest_collection_modifyitems socketserver hook. Verify every HTTP test carries @requires_loopback explicitly first. Keep the helper / marker re-exports so legacy imports stay working.
3. Cleanup: delete the three leftover audit files (CLUSTER_AUDIT_PR5.md, CLUSTER_AUDIT_PR8.md, CLUSTER_AUDIT_PR9.md) and run a final lint pass.

Out of scope: golden_corpus/, the long tail of feature-specific top-level files, and any source code under xrspatial/geotiff/.

Definition of done

• pytest xrspatial/geotiff/tests/ -x -q passes.
• unit/ holds the consolidated header / geotags / safe-xml / compression / dtype tests.
• conftest.py no longer carries the socketserver hook.
• Three audit files removed.
• File count drops further toward the epic target.

This PR closes epic #2390.