Add check_image_series_external_file_forward_slashes

[bendichter]

Closes #697

Summary

Adds a check that flags backslashes in ImageSeries.external_file paths. These are produced by platform-dependent path joining on Windows (e.g. ses-1_image\abc123_external_file_0.mp4) and silently fail to resolve on POSIX systems and against archive asset keys, which always use forward slashes. This is what caused get_dandi_video_info() to return empty results for Dandiset 001771.

The check decodes bytes paths like the neighboring checks do, and yields one message per offending entry so a series with several external files reports each bad one.

Severity

Registered at BEST_PRACTICE_VIOLATION, paralleling the sibling check_image_series_external_file_relative. A path that is genuinely broken locally is already reported as CRITICAL by check_image_series_external_file_valid. Happy to raise this to CRITICAL if reviewers feel the silent failure on the archive warrants it.

Changes

• New check_image_series_external_file_forward_slashes in src/nwbinspector/checks/_image_series.py, exported from checks/__init__.py
• New best-practice section in docs/best_practices/image_series.rst under the best_practice_image_series_external_file_forward_slashes anchor referenced by the docstring
• CHANGELOG entry under "New Checks"

Testing

Five new unit tests cover the passing forward-slash path, a non-external series, the backslash trigger, a bytestring path, and multiple external files. All tests in test_image_series.py pass; ruff and black are clean. Also ran end-to-end through inspect_nwbfile on a written NWB file containing one backslash and one forward-slash ImageSeries — the check fires on the backslash one only.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.12%. Comparing base (9be6250) to head (fbaeac7).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #717      +/-   ##
==========================================
+ Coverage   79.71%   83.12%   +3.40%     
==========================================
  Files          48       48              
  Lines        1923     1932       +9     
==========================================
+ Hits         1533     1606      +73     
+ Misses        390      326      -64     

Files with missing lines	Coverage Δ
src/nwbinspector/checks/__init__.py	100.00% <ø> (ø)
src/nwbinspector/checks/_image_series.py	96.00% <100.00%> (+0.87%)	⬆️

... and 5 files with indirect coverage changes

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