[LucaMantani]

resourcebuilder.check_types (and the parse_/allowed_keys checks in configparser) call isinstance(value, annotation) directly. That breaks with three common annotation patterns:

1. @explicit_node resources. When a provider parameter is filled by an @explicit_node produce_X, the namespace still holds the ExplicitNode wrapper at check time (_process_requirement turns its .value into a node of its own). The real value doesn't exist yet, so any correct annotation fails:
   BadInputType: Value '<ExplicitNode ...>' is not of type Union, but of type 'ExplicitNode'.
   Provider outputs are never checked at this stage, so this change skips ExplicitNode values in the same way.
2. Parametrised generics. def provider(settings: Mapping[str, Any]) crashes graph building with a bare TypeError: isinstance() argument 2 cannot be a parameterized generic, without naming the provider or parameter.
3. X | None on Python 3.10–3.13. When the check fails, BadInputType itself raises AttributeError: 'types.UnionType' object has no attribute '__name__'.

Changes

• New configparser.check_annotation(val, tp). It behaves like isinstance for classes and tuples, matches unions if any member matches, and checks generics against their origin class (type arguments are not validated). It's used by check_types, _parse_func and check_key_is_allowed.
• check_types skips ExplicitNode values.
• BadInputType names unions and generics via str(tp).
• Tests in test_builder.py (explicit node + generic/union provider annotations, origin mismatch) and test_configparser.py (generic/union parse_ inputs). All three fail on master.

The full suite passes on Python 3.11 and 3.14, except test_collect and test_parallel_execute. Those two fail identically on master: recent distributed removed Client.register_worker_plugin.

Context: found in smefit, where adding type hints to a provider broke the graph.

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