Flatten Enum members to bare values on the serialize path

[elinscott]

Problem

aiida-pythonjob's general_serializer has no serializer for Enum members. 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 general_serializer, 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.

This is a one-way flattening on the write path, not a round-trip. Under the pinned node-graph (0.6.5) the read side (coerce_inputs_from_spec) reconstructs only structured models — dataclass/pydantic/TypedDict — and records no structured_type extra for Enum sockets, so it never rebuilds the member. Task bodies therefore receive the bare .value, not the Enum they declared. This is harmless for IntEnum / str, Enum consumed by value or equality, but a body that tests x is Color.RED / x == Color.RED against a plain Enum will see False. True reconstruction would need an enum branch in node-graph's structured_type_info/coerce_structured_value and is left as follow-up there.

Two edge cases are handled explicitly: set/frozenset are left untouched (a set fails in general_serializer regardless of its contents — not JSON-serializable, no registered serializer — so descending into one to flatten enums would not make it serializable; documented as a limitation), and dict-key flattening now raises on a collision (two distinct keys collapsing to one, e.g. an Enum member and its bare value) rather than silently dropping an entry.

Testing

New unit tests in tests/test_serializer.py, including test_body_receives_bare_value_not_member, which drives a real wg.run() and asserts a plain-Enum input reaches the body as the bare value — pinning the actual (non-round-trip) behaviour so it flips loudly if node-graph later adds enum reconstruction — plus tests for the set passthrough and the key-collision guard.

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 (the bodies receive the bare enum value, per the note above).