Fix ClassInitError crashing instead of reporting a missing class

[jacquelinegarrahan]

Fix ClassInitError crashing instead of reporting a missing class

Branch: bugfix/classiniterror-message → main

Bug

Two coupled defects in HDF5Serializer.deserialize's error handling
(lume/serializers/hdf5.py, lume/serializers/base.py).

(a) The intended error is dead code. Line 80 was:

object_type = getattr(object_import_module, object_name)   # no default
if not object_type:
    raise ClassInitError(object_name, object_import_module, package_version)

getattr without a default raises AttributeError immediately when the class
is missing, so the if not object_type / ClassInitError branch could never
run. Deserializing an archive that names a class not present in the installed
package produced a bare AttributeError instead of the helpful, version-aware
ClassInitError.

(b) The error class crashes when constructed. ClassInitError.__init__
(and ModuleImportError.__init__) did module_name.split(".")[0], but the
caller passed the imported module object, not a string:

raise ClassInitError(object_name, object_import_module, package_version)
#                                  ^^^^^^^^^^^^^^^^^^^^ a module, not a str

So even if the branch were reached, building the exception raised
AttributeError: module '...' has no attribute 'split', masking the original
failure entirely.

Reproduction (before):

raise ClassInitError("Foo", some_module, "1.0")
# AttributeError: module '...' has no attribute 'split'

Fix

• In deserialize, use getattr(..., None) and check is None, so the
  ClassInitError path actually fires for a missing class; pass the module
  name string (package_str).
• Extract the base package name via a small _module_base helper that accepts
  either a dotted string or a module object, so the error path is robust
  regardless of caller.

Verification

pytest -q                                 # 121 passed
ClassInitError("Foo", "lume.x", "1.0")    # message OK (string arg)
ClassInitError("Foo", lume, "1.0")        # message OK (module arg, was crash)
deserialize(<archive naming a missing class>)  # -> ClassInitError (was AttributeError)

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