BUG: Fix HDFStore.put with StringDtype columns and compression (#64180)

[alubbock]

In GenericFixed.write_array, when compression filters are set, Atom.from_dtype(value.dtype) is called to determine the HDF5 atom type. StringDtype is an ExtensionDtype that lacks the .base attribute Atom.from_dtype expects, raising AttributeError.

Route BaseStringArray to the VLArray path before the Atom.from_dtype logic runs, and pass self._filters through to create_vlarray so that compression is still applied.

AI-assisted (Claude Code), but essentially it's just a reorganisation of the branching logic. A test has been added, and I've verified that the fix works with the example in #64180.

• closes BUG: to_hdf on dataframe with string column failing with compression #64180
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