Add num_samples to ImageSeries

[rly]

Summary

• Points the nwb-schema submodule to the latest dev tip (489cc31f), which adds the optional
  num_samples dataset to ImageSeries. Corresponds to Add optional num_samples dataset to ImageSeries for external file timing nwb-schema#678
• Implements num_samples in ImageSeries: when format='external' and rate-based timing is
  used, the data array is empty (shape (0, 0, 0)) and its length cannot be used to determine the
  frame count. num_samples fills that gap.
• Raises ValueError on construction when external_file + rate are set but num_samples is
  absent. Files missing the field are read silently (no warning) to avoid noise from existing data.
• Overrides TimeSeries.num_samples (a computed read-only property) with a stored attribute backed
  by self._num_samples, falling back to the parent implementation for data-bearing series.
• Updates gallery docs to document the rate + num_samples requirement for external-file series.
• Pins HDMF to >=6.0.2, which fixed a TypeError when writing np.uint64 values.

Checklist

• num_samples is written to and read back from HDF5 (roundtrip test added)
• ValueError raised when external_file + rate + no num_samples on construction
• No error or warning when reading old files without num_samples
• num_samples not required when timestamps is provided
• Gallery docs updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.24%. Comparing base (2bdc540) to head (43e3136).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2194      +/-   ##
==========================================
+ Coverage   95.21%   95.24%   +0.02%     
==========================================
  Files          30       30              
  Lines        2989     3005      +16     
  Branches      442      446       +4     
==========================================
+ Hits         2846     2862      +16     
  Misses         87       87              
  Partials       56       56              

Flag	Coverage Δ
integration	73.21% <75.00%> (+0.14%)	⬆️
unit	85.65% <87.50%> (+<0.01%)	⬆️

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