Fix external-file ImageSeries test and require pynwb>=4.0

[rly]

Follow-up to #707.

Problem

After PyNWB 4.0.0 was released, the daily and dev CI workflows started failing on tests/unit_tests/test_image_series.py::TestExternalFileValid::test_check_image_series_external_file_valid_bytestring_pass:

ValueError: ImageSeries 'TestImageSeries': num_samples should be set when format='external'
and rate is used for timing, because data is empty and its length cannot be used to determine
the number of frames.

PyNWB 4.0.0 added validation that an ImageSeries with format='external' and rate timing must set num_samples, since the frame count cannot be inferred from the (empty) data length. The test constructs exactly such an ImageSeries without num_samples, so it now raises at construction time.

#707 ran against PyNWB 3.1.3 (PyNWB 4.0.0 had not yet been released), so it did not surface this. The scheduled runs picked up 4.0.0 once it was published.

Changes

• Pass num_samples=1 when constructing the ImageSeries in that test, satisfying the PyNWB 4.0.0 validation.
• Raise the minimum PyNWB to >=4.0 in pyproject.toml. NWB Inspector targets the latest PyNWB only (see Support only latest PyNWB #510), and the latest release is now 4.0.0, so the floor is bumped to match.

Scope

The test is the only place in the suite combining format="external" with rate timing and empty data. Other external-file constructions use timestamps, which derive the frame count and do not trip the new check.

Note: the test-dandi-dev / test-dandi-dev-live jobs fail with the same ValueError raised from DANDI's own test fixtures (dandi/tests/fixtures.py), which is an upstream DANDI incompatibility with PyNWB 4.0 and is not addressed here.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.81%. Comparing base (13d0d67) to head (7c57e8a).
⚠️ Report is 5 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #708      +/-   ##
==========================================
+ Coverage   79.44%   82.81%   +3.37%     
==========================================
  Files          48       48              
  Lines        1897     1897              
==========================================
+ Hits         1507     1571      +64     
+ Misses        390      326      -64     

see 5 files with indirect coverage changes

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