Preserve JSON Schema metadata on annotated Struct union members - #1213

Summary

msgspec.json.schema already keeps extra JSON Schema from msgspec.Meta on non-struct union members (for example Annotated[None, Meta(description=...)]). The same metadata is dropped when it is attached to a Struct.

The union walker unwraps Metadata to classify each member, then previously stored only the inner StructType. Schema generation therefore never saw the annotation. This stores the original union member alongside the unwrapped type and uses that member when emitting anyOf.

Reproduced on current main:

from typing import Annotated, Union
import msgspec

class Point(msgspec.Struct):
    x: int

PointWithMeta = Annotated[Point, msgspec.Meta(description="a point")]
print(msgspec.json.schema(Union[PointWithMeta, None])["anyOf"])
# before: [{'$ref': '#/$defs/Point'}, {'type': 'null'}]
# after:  [{'$ref': '#/$defs/Point', 'description': 'a point'}, {'type': 'null'}]

The same loss happens for tagged struct unions.

Tests

• tests/unit/test_schema.py::test_optional_union_struct_member_metadata_preserved
• tests/unit/test_schema.py::test_struct_tagged_union_member_metadata_preserved

pytest tests/unit/test_schema.py: 126 passed, 8 skipped.