Complete TestAddImaging tests for the dict-based metadata pipeline

[h-mayorquin]

Completes the tests that were missing from #1677 to keep that diff focused on the implementation. The remaining tests are added here in the new pytest format so we can drop TestAddPhotonSeries entirely once we drop support for the list-based metadata format (September 2026).

[pauladkisson]

Looks good to me!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.01%. Comparing base (b630a43) to head (0ce7c14).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1685      +/-   ##
==========================================
- Coverage   81.08%   81.01%   -0.07%     
==========================================
  Files         160      160              
  Lines       12574    12589      +15     
==========================================
+ Hits        10195    10199       +4     
- Misses       2379     2390      +11     

Flag	Coverage Δ
unittests	81.01% <ø> (-0.07%)	⬇️

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Files with missing lines	Coverage Δ
...nv/tools/nwb_helpers/_metadata_and_file_helpers.py	91.81% <ø> (ø)
src/neuroconv/tools/roiextractors/roiextractors.py	75.90% <ø> (-1.14%)	⬇️

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