geotiff tests: consolidate unit cluster + slim conftest, closes #2390

[brendancol]

Closes #2414
Closes #2390

Final PR of the GeoTIFF test consolidation epic. Three deliverables.

Unit-level consolidation

New xrspatial/geotiff/tests/unit/ directory holds the pure-unit
helper tests:

• unit/test_header.py — relocated from test_header.py
• unit/test_geotags.py — relocated from test_geotags.py
• unit/test_safe_xml.py — relocated from test_gdal_metadata_xml_escape_1614.py
• unit/test_compression.py — merged from test_packbits_jit_2048.py and
  test_packbits_jit_2049.py

Files that bundle a unit slice with end-to-end IO (test_lz4.py,
test_lerc.py, test_jpeg.py, test_jpeg2000.py,
test_mixed_bps.py, test_mixed_sample_format.py,
test_compression_level.py, test_compression_docstring_1644.py,
test_extra_tags_safe_filter_1657.py, test_lerc_max_z_error.py)
stay where they are. Splitting them is a future-epic problem.

The release-gate row for writer.gdal_metadata_xml in
docs/source/reference/release_gate_geotiff.rst was updated to cite
the relocated unit/test_safe_xml.py.

conftest slim + explicit @requires_loopback

Dropped the pytest_collection_modifyitems socketserver hook in
xrspatial/geotiff/tests/conftest.py. Every HTTP/loopback test
across the suite now carries @requires_loopback explicitly. Markers
added on:

• parity/test_backend_matrix.py — the matrix and error-matrix tests
• parity/test_pixel_equality.py — the two miniswhite HTTP tests
• test_golden_corpus_http_1930.py — module-level pytestmark
• test_read_geotiff_gpu_url_eager_2161.py — three HTTP cases
• test_remote_sidecar_byte_order_2314.py — four HTTP cases
• test_remote_sidecar_chunked_2239.py — four HTTP cases
• test_sidecar_max_cloud_bytes_2121.py — three HTTP cases
• test_sidecar_ovr_2112.py — four HTTP cases
• test_parallel_strip_decode_2100.py — the HTTP class plus one outside
• test_parallel_strip_decode_sparse_2100.py — the HTTP class
• write/test_cog.py — rows 5 and 6

The conftest still re-exports make_minimal_tiff and the marker
names so legacy from .conftest import ... keep resolving.

Cleanup

Deleted three stale audit files (CLUSTER_AUDIT_PR5.md,
CLUSTER_AUDIT_PR8.md, CLUSTER_AUDIT_PR9.md) that leaked into
main from prior PRs.

Verification

• pytest xrspatial/geotiff/tests/unit/ — 135 passed
• pytest xrspatial/geotiff/tests/ — 5719 passed, 68 skipped, 6 xfailed
• grep -n "pytest_collection_modifyitems\|socketserver.TCPServer\|_serve(" conftest.py — empty
• Every file that uses socketserver or _serve( also carries
  @requires_loopback.
• Loopback-denied simulation (force _HAS_LOOPBACK=False) skips the
  affected tests cleanly with no errors.

Epic-wide file-count delta

PR	Cluster	Top-level removed	Files added
#2394	VRT missing-sources	2	1 (vrt/test_missing_sources.py)
#2404	VRT validation	3	1 (vrt/test_validation.py)
#2405	Attrs contract	4	1 (attrs/test_contract.py)
#2406	Rotated / dropped CRS	3	1 (read/test_crs.py)
#2407	VRT metadata / window / dtype	~120	3 (vrt/test_{metadata,window,dtype_conversion}.py)
#2408	Backend parity	~25	2 (parity/test_{backend_matrix,pixel_equality}.py)
#2409	Release-gate registry	n/a	1 (release_gates/test_stable_features.py)
#2410	Writer / COG / BigTIFF / overview	~20	4 (write/test_*.py)
#2411	Integration cluster	~20	3 (integration/test_*.py)
#2412	Reader paths	~30	9 (read/test_*.py)
this	Unit cluster + conftest slim	5	4 (unit/test_*.py)

Total cluster reduction: from 354 files at epic start to 253 today
(excluding golden_corpus/).

Test plan

• pytest xrspatial/geotiff/tests/unit/ passes
• pytest xrspatial/geotiff/tests/ passes under loopback-available
• Suite passes under simulated loopback-denied (markers skip cleanly)
• No file in the suite uses socketserver or _serve( without
  also carrying @requires_loopback
• Conftest no longer references pytest_collection_modifyitems
• Stale audit files removed
• CLUSTER_AUDIT_PR11.md is deleted in a follow-up commit on
  this branch before merge

[brendancol]

PR Review: geotiff tests: consolidate unit cluster + slim conftest

PR 11 of epic #2390. The diff lands cleanly: pure-unit files relocate under unit/, the socketserver hook in conftest.py is replaced by explicit @requires_loopback markers on every HTTP test, and three stale audit files are removed. Verification I reproduced locally:

• pytest xrspatial/geotiff/tests/unit/ — 135 passed
• pytest xrspatial/geotiff/tests/ — 5719 passed, 68 skipped, 6 xfailed
• grep -n "pytest_collection_modifyitems\|socketserver.TCPServer\|_serve(" conftest.py — empty
• grep -rL "requires_loopback" $(grep -rl "socketserver\|_serve(" xrspatial/geotiff/tests/) — empty

Blockers

None.

Suggestions

None.

Nits

• xrspatial/geotiff/tests/CLUSTER_AUDIT_PR11.md is the audit deliverable and is expected to disappear in a follow-up commit on this branch before merge (per the epic contract). Prior PRs missed this step; the final commit on this branch should drop the file. The PR body already promises this; flagging here so it does not slip again.
• xrspatial/geotiff/tests/unit/test_safe_xml.py is a renamed file with zero diff (the rename is the entire change). The line count in git show --stat shows 0 additions, 0 deletions. That is expected — surfacing it so reviewers reading the file list do not look for missing content.

What looks good

• The unit/ scope is correctly conservative. Mixed files (test_lz4.py, test_lerc.py, test_jpeg.py, test_jpeg2000.py, test_compression_level.py, etc.) stay where they are; the audit table records why.
• The @requires_loopback placement matches the loopback usage. Tests that only bind via a fixture (parity/test_backend_matrix.py::test_backend_parity_matrix*) get the marker; tests that ignore the HTTP fixture in the same module are left alone.
• The conftest still re-exports make_minimal_tiff and the marker names under __all__, so existing from .conftest import ... imports across the suite keep resolving without a churn pass.
• The release-gate rst row for writer.gdal_metadata_xml was updated to cite the new path, so the existing test_release_gate_cites_only_existing_test_files check stays green.
• The two import io unused imports and one E402-shaped import in test_read_geotiff_gpu_url_eager_2161.py introduced by the marker addition have been cleaned up; the only remaining E402 warnings on touched files are pre-existing in parity/test_backend_matrix.py and were not introduced by this PR.

Checklist

• Algorithm correctness: N/A (tests-only refactor)
• Backend parity: N/A
• NaN handling: N/A
• Edge cases: marker placement audited test-by-test
• Dask correctness: N/A
• No premature materialization or copies: N/A
• Benchmark: N/A
• README feature matrix: N/A
• Docstrings: existing docstrings preserved on move

[brendancol]

Follow-up review

Fix disposition of the previous round:

• Nit 1 — fixed. CLUSTER_AUDIT_PR11.md has been deleted in commit c879ce47 (the final commit on this branch). The audit no longer leaks into main on merge.
• Nit 2 — dismissed (informational only). The zero-diff renamed unit/test_safe_xml.py is intentional; no action needed.

No new findings. PR is clean to merge.