Drop xfail on test_external_cog_validator now that #2308 is fixed

[brendancol]

Follow-up to #2304 and #2309.

#2304 wired up the rio-cogeo CI gate and marked test_external_cog_validator as xfail (strict=False) because the writer emitted overview tile blocks in the wrong order (#2308). #2309 fixed the writer, so the test now passes locally with XRSPATIAL_REQUIRE_COG_VALIDATOR=1. Drop the marker and the comment block that pointed at #2308.

Verified locally with rio-cogeo installed: the test goes from XPASS (silent under strict=False) to PASS. Final validation is the run (3.12) pytest-cog-validator job on this PR.

Diff is the decorator plus its preceding comment block, nothing else.

[brendancol]

Self-review

Diff is 16 deletions: the @pytest.mark.xfail decorator on test_external_cog_validator and the preceding comment block that pointed at #2308. Test body is unchanged.

Verified locally with rio-cogeo installed:

XRSPATIAL_REQUIRE_COG_VALIDATOR=1 pytest \
  xrspatial/geotiff/tests/test_cog_writer_compliance.py::test_external_cog_validator -v
...
xrspatial/geotiff/tests/test_cog_writer_compliance.py::test_external_cog_validator PASSED

Blockers

• none

Suggestions

• none

Nits

• none

Checklist

• Behavior change is the decorator removal only
• Test passes locally under the strict env var
• No other files touched
• CI run (3.12) pytest-cog-validator job is the final gate