Restore ValueError expectation in test_generator_ccds_without_unit

[mwcraig]

Follow-up to a Copilot review comment on #925 (merged), which pointed out that test_generator_ccds_without_unit now carries both the backend_xfail("jax") marker (added in cf4f4b7) and an explicit jax branch asserting that ccds() returns a CCDData for unit-less files.

Those two contradict each other. The marker's reason documents that the jax behavior is platform-dependent: on Linux CI ccds() fails to raise ValueError, while on macOS with identical astropy/jax versions it does raise. The jax branch added in #925 asserts the Linux behavior unconditionally, so on macOS it fails and is silently absorbed by the marker as an xfail; on Linux it passes and is reported as a (non-strict) xpass. Net effect: the test can never fail on jax on any platform, and on macOS it asserts the wrong thing.

This PR reverts the test body to expect ValueError on all backends and keeps the backend_xfail marker. Raising for unit-less files is the intended behavior; the unexplained Linux-only jax deviation stays visible as an xfail rather than being encoded as correct. (Copilot suggested removing the marker instead, but that would break macOS jax runs given the platform dependence.)

Verified locally: test_image_collection.py passes under numpy and dask; under jax on macOS the reverted test xpasses (non-strict), and on Linux CI it should xfail as it did before #925.

Checklist

• Did you add an entry to the CHANGES.rst file? (Not needed — test-only change with no user-facing behavior.)
• Did you add a regression test? (Test-only change.)

🤖 Generated with Claude Code

https://claude.ai/code/session_01463EbSeiTmBJevbvpMNRm8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.63%. Comparing base (8064a74) to head (b932212).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #961   +/-   ##
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  Coverage   96.63%   96.63%           
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  Files           8        8           
  Lines        1574     1574           
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  Hits         1521     1521           
  Misses         53       53           

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