Keep Variance/InverseVariance uncertainty propagation in the array namespace

[mwcraig]

_VarianceUncertaintyWrapper and _InverseVarianceWrapper in ccdproc/_ccddata_wrapper_for_array_api.py were bare subclasses, so arithmetic on a CCDData carrying a VarianceUncertainty / InverseVariance went through astropy's default to_variance / from_variance hooks and the propagated uncertainty escaped to NumPy on JAX (and other non-NumPy backends).

This PR factors the four _propagate_add/subtract/multiply/divide overrides that _StdDevUncertaintyWrapper already had into a small _ArrayAPIPropagationMixin; each wrapper supplies namespace-aware hooks via _variance_hooks(xp):

• StdDev: xp.square / xp.sqrt (unchanged behaviour)
• Variance: identity / xp.asarray
• InverseVariance: 1/x / 1/xp.asarray(x)

The two jax backend_xfail params on test_cosmicray_gain_correct (VarianceUncertainty and InverseVariance with gain_apply=True) are removed; under jax they go XPASS -> PASS.

Note: intermediates may still round-trip through NumPy inside astropy's _VariancePropagationMixin itself (it calls np.sqrt / np.abs and checks isinstance(correlation, np.ndarray)); that is upstream and tracked in #940. This PR only fixes the ccdproc-side wrapper so the result stays in the data's namespace.

Test matrix

backend	pytest ccdproc
numpy	380 passed, 5 skipped
jax (JAX_ENABLE_X64=True)	368 passed, 10 skipped, 7 xpassed (same 7 XPASS as on main; 2 fewer xfails — the two removed here)
dask	370 passed, 15 skipped
dask + CCDPROC_ENFORCE_ESCAPE_BASELINE=1	370 passed, 15 skipped

ccdproc/tests/test_cosmicray.py under array-api-strict: 1 failed, 7 passed, 33 xfailed both before and after (pre-existing failure, unrelated).

Fixes #962

🤖 Generated with Claude Code

https://claude.ai/code/session_01DTN9DnPnLKK2u7knnMJ2gA

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.65%. Comparing base (c2ce2e2) to head (60d9004).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ccdproc/_ccddata_wrapper_for_array_api.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #975      +/-   ##
==========================================
+ Coverage   96.13%   96.65%   +0.51%     
==========================================
  Files           8        8              
  Lines        1655     1672      +17     
==========================================
+ Hits         1591     1616      +25     
+ Misses         64       56       -8     

Flag	Coverage Δ
dask	95.80% <100.00%> (+0.58%)	⬆️
jax	95.92% <100.00%> (+0.58%)	⬆️
numpy	96.53% <95.00%> (+0.51%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.