Re-enable xarray timedelta/datetime roundtrip tests on xarray >= 2026.7.0

[li-em]

#2229 skipped the xarray timedelta/datetime roundtrip tests because no released
xarray was compatible with numpy >= 2.5 time-coding (int64 * timedelta64
overflow, datetime decode failures, and a generic-unit timedelta
DeprecationWarning raised at import).

xarray 2026.7.0 ships the fix, so this re-enables the tests — but only where
the fix exists. Our minimum supported xarray (2025.07.1) still hits all of the
above when installed alongside current numpy, which is exactly what the
minimum-versions CI matrix does. A plain revert of #2229 therefore fails those
jobs (54 xarray-backends failures + 4 collection errors in test_dask.py).

Changes:

• run_xarray_backends_tests.py: the three skip overrides
  (test_roundtrip_timedelta_data, test_roundtrip_timedelta_data_via_dtype,
  test_roundtrip_numpy_datetime_data) are now defined only when
  xarray < 2026.7.0. On newer xarray the upstream tests run for real.
• pyproject.toml: the generic-timedelta DeprecationWarning filter stays
  (old xarray raises it on import under numpy >= 2.5); it's inert on newer
  xarray.

Both can be dropped once the minimum supported xarray reaches 2026.7.0.

Closes #2230

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.70%. Comparing base (4615c81) to head (122f0dd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2297   +/-   ##
=======================================
  Coverage   84.70%   84.70%           
=======================================
  Files          88       88           
  Lines       38995    38995           
=======================================
  Hits        33030    33030           
  Misses       5965     5965           

Flag	Coverage Δ
python	90.06% <ø> (ø)
rust	84.10% <ø> (ø)

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