Flatten Enum members to bare values on the serialize path

[elinscott]

Problem

node_graph declares that an enum-typed socket's serialized form is the bare member value: socket_spec records structured_type extras precisely so coerce_inputs_from_spec can rebuild the Enum member before a task body runs.

The AiiDA serialization adapter never honoured that contract on the write path: a raw Enum instance (or a dict/list containing one, e.g. a parameters dict with an enum-valued entry) flows straight into serialize_ports, which hands it to aiida-pythonjob's general_serializer — which has no serializer for enums — and the whole submission fails at submit time with an opaque serialization error.

Change

A small _flatten_enums helper replaces Enum members with their .value, recursively through dicts (keys and values), lists and tuples, and is applied in both AiidaSerializationAdapter.serialize and AiidaSerializationAdapter.serialize_ports immediately after resolve_tagged_values. The isinstance(value, Enum) check also matches wrapt proxies (TaggedValue) wrapping enum members, so tagged graph inputs are handled by the same path.

Round-trip is preserved: the read side (coerce_inputs_from_spec in node-graph) rebuilds the member from the bare value using the recorded structured_type, so task bodies still receive the Enum they declared.

Testing

New unit tests in tests/test_serializer.py

Exercised end-to-end by the downstream Koopmans package, whose workgraphs pass enum-valued settings (e.g. functional and spin-treatment enums) through @task.graph inputs: before the change every such submission failed in general_serializer; after it, the graphs submit and the bodies receive rebuilt members.