Allow per-call CCD extension overrides

[nomad3]

Summary

• add an optional ext argument to ImageFileCollection.ccds() for selecting a FITS data extension per call
• keep the collection's own extension unchanged for primary-header summaries and filtering
• continue supporting ccd_kwargs["hdu"], avoid mutating caller dictionaries, and reject contradictory selectors explicitly
• cover integer, extension-name, (name, version), and zero-valued selectors with offline multi-extension FITS regressions
• correct the generated NumPy-style parameter indentation so the ccds() docstring builds cleanly under Sphinx warnings-as-errors

Compatibility boundary: ext is now reserved as the per-call selector; code that needs to filter an EXT header keyword should filter the collection before calling ccds().

Fixes #817

Validation

• initial override/conflict regression on untouched main: 7 failed
• python -m pytest ccdproc/tests/test_image_collection.py -q — 85 passed
• full NumPy suite — 354 passed, 29 skipped
• full Dask suite — 349 passed, 34 skipped
• full JAX x64 suite — 347 passed, 29 skipped, 7 established xfails
• inspected the final cleaned docstring: ext is a parameter and every description line is indented beneath it
• Black, Ruff, git diff --check, and git show --check

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Checklist

• For new contributors: Did you add yourself to the AUTHORS.rst file? (Present on current main through merged Exclude masked weights from average combinations #952.)
• For documentation changes: Does the commit message include a [skip ci]? (Not a documentation-only change.)

For new functionality:

• Did you add an entry to CHANGES.rst?
• Did you include a meaningful docstring with Parameters?
• Does the commit message include Fixes #817?
• Did you include tests for the new functionality?
• Does this PR add, rename, move, or remove an existing function or parameter? (It adds the optional ext parameter.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.64%. Comparing base (e2d43f3) to head (81fe53d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #960      +/-   ##
==========================================
+ Coverage   96.63%   96.64%   +0.01%     
==========================================
  Files           8        8              
  Lines        1575     1581       +6     
==========================================
+ Hits         1522     1528       +6     
  Misses         53       53              

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