fix: export scalar datetime as a scalar dataset

[ehennestad]

Motivation

Problem

When a scalar datetime property such as session_start_time is set, types.util.correctType ran num2cell on it, converting the scalar datetime into a 1×1 cell array before formatting. On export that 1×1 cell was written to HDF5 as a 1-element, non-scalar dataset instead of a scalar dataset.

So a value that is conceptually scalar was stored with the wrong dataspace. The exported file no longer reflected the field's scalar shape, the value did not round-trip faithfully (a scalar went in, a 1-element array came back), and on read it was returned as a lazy DataStub instead of an eagerly-loaded scalar.

Solution

Drop the num2cell conversion and format datetimes in place, so the datetime branch of correctType preserves the class and shape of its input:

• a scalar datetime stays a scalar datetime
• a datetime array stays a datetime array (formatted element-wise)
• a cell array of datetimes stays a cell array of datetimes

Scalar datetime properties are now written as scalar datasets. Covered by a test asserting the scalar dataspace on export, plus an updated reader test reflecting that a scalar dataset is read eagerly rather than as a DataStub.

How to test the behavior?

% session_start_time is a scalar datetime
nwb = tests.factory.NWBFile();
nwbExport(nwb, "scalar_datetime.nwb");

info = h5info("scalar_datetime.nwb", "/session_start_time");
disp(info.Dataspace.Type)   % "scalar"  (previously a 1-element, non-scalar dataset)

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.31%. Comparing base (6f4e97a) to head (f953063).

Additional details and impacted files

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  Coverage   95.31%   95.31%           
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  Files         219      219           
  Lines        7647     7647           
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  Hits         7289     7289           
  Misses        358      358           

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