Correctly handle HDF5 fillvalue for string dtype arrays.

[sharkinsspatial]

What I did

Updated the HDF5 parser to support string dtype arrays with a bytes or str fillvalue.

Acceptance criteria:

• Closes Failure to read HDF5 data with data arrays of dtype='<U1' #878
• Tests added
• Tests passing
• No test coverage regression

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.93%. Comparing base (2f04001) to head (9ed7c07).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
virtualizarr/parsers/hdf/hdf.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #988      +/-   ##
==========================================
- Coverage   89.97%   89.93%   -0.04%     
==========================================
  Files          33       33              
  Lines        2064     2077      +13     
==========================================
+ Hits         1857     1868      +11     
- Misses        207      209       +2     

Files with missing lines	Coverage Δ
virtualizarr/codecs.py	93.44% <100.00%> (+0.22%)	⬆️
virtualizarr/parsers/hdf/hdf.py	94.55% <84.61%> (-1.04%)	⬇️

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[maxrjones]

Suggested change

	if dtype.kind == "O":
	dtype = np.dtypes.StringDType()
	# Discriminate against other object-kind HDF5 dtypes (vlen arrays, object/
	# region references) that would silently be coerced to StringDType and
	# produce garbage downstream — those aren't supported yet, so fail loudly.
	if h5py.check_string_dtype(dtype) is not None:
	dtype = np.dtypes.StringDType()
	elif dtype.kind == "O":
	raise NotImplementedError(
	f"HDF5 object dtype {dtype!r} is not a variable-length string and "
	f"is not yet supported by HDFParser. h5py exposes vlen arrays "
	f"(`h5py.vlen_dtype`) and object/region references "
	f"(`h5py.ref_dtype`, `h5py.regionref_dtype`) as numpy object dtype; "
	f"please open an issue if your file needs one of these."
	)

I don't think all hdf5 object-kind dtypes are variable-length strings