Add consolidated VRT missing_sources policy matrix tests (#2367)

[brendancol]

Closes #2367. Sub-task of epic #2342.

Adds a single test file that pins the four-by-two policy matrix for VRT missing_sources:

• default / explicit 'raise' / 'warn' / invalid value
• each exercised against eager read_vrt and dask open_geotiff(.vrt, chunks=...)

Assertions cover the exception type and message (raise / invalid), the GeoTIFFFallbackWarning class plus message text (warn), and the actual output array values (NaN on the missing half, fill value on the present half).

The existing tests at test_vrt_missing_sources_policy_1799.py, test_vrt_chunked_missing_sources_1799.py, test_vrt_missing_sources_default_raise_1843.py, and test_read_vrt_default_missing_sources_1860.py each pin a slice of this matrix. They stay in place; this file keeps the full matrix together so a kwarg refactor that drops eager/dask parity regresses one focused file.

Test plan

• pytest xrspatial/geotiff/tests/test_vrt_missing_sources_policy_2367.py (18 passed locally)
• Re-ran the four sibling test files; all still pass.

[brendancol]

PR Review: VRT missing_sources policy matrix tests (#2367)

Blockers

None.

Suggestions

None.

Nits

• test_vrt_missing_sources_policy_2367.py:113-117: the eager warn test uses pytest.warns(GeoTIFFFallbackWarning) as record and re-checks the message in the body instead of passing match=. Either style works; flagging only because the sibling 1799 test goes with match=.
• test_vrt_missing_sources_policy_2367.py:262-269: nice parametrize set on the bad values ('ignore', 'RAISE', 'raises', empty, trailing space, '1'). The value-validation block at vrt.py:399-402 runs before chunked dispatch, so the chunked invocations hit the same error path, which is the cross-mode parity this matrix is meant to pin.

What looks good

• Four-by-two matrix in one file: default / explicit-raise / warn / invalid against both eager and dask readers.
• Output value assertions on the warn path (NaN on the missing half, 7.0 on the present half) under both readers, not just "did it warn".
• Warning class is checked as GeoTIFFFallbackWarning, not bare UserWarning.
• Build-time vrt_holes is asserted on the dask warn path so callers can branch on partial mosaics without forcing a compute.
• Temp filenames all carry _2367 so parallel runs don't collide.
• The parametrize-by-reader pattern keeps eager and dask symmetric for the raise / invalid cases without duplicating the test bodies.

Checklist

• Test-only change; algorithm correctness not applicable
• Both supported read paths exercised (eager read_vrt, dask open_geotiff(.vrt, chunks=))
• NaN handling on the missing region asserted
• Edge cases on the bad-value side (empty string, trailing space, wrong case) covered
• Dask path's parse-time vrt_holes and compute-time warning both pinned
• No performance-critical paths touched
• No new public API; README / docs untouched on purpose
• Docstrings on every test class explain what they pin and why