Generate OpenAPI schemas for unions on Python 3.14

[davegaeddert]

plain api generate-openapi raises ValueError: Unknown type: str | None on Python 3.14, so any view with an optional field in its response schema produces no document and the served /api/openapi.json 500s. Shipped in 0.36.0; found tonight by plainframework.com's own upgrade, which held its deploy over it.

Cause. Python 3.14 unified types.UnionType with typing.Union, so (str | None).__origin__ now exists and is typing.Union where on 3.13 it raised AttributeError. schema_from_type() probed hasattr(t, "__origin__"), handled list and dict, and raised on anything else — and the branch that handles X | None sat after it. On 3.13 unions fell through to the right branch; on 3.14 they're caught and rejected.

Fix. Match unions explicitly and first, and use typing.get_origin/get_args rather than probing dunder attributes, which is the actual root cause: attribute probing on typing internals is what broke across a Python release. X | None keeps byte-identical output (member schema plus nullable: true), so no document that generated on 3.13 changes. A union of several named members now emits anyOf plus nullable when None is among them, instead of raising — str | int is a legal annotation, and taking down the whole document over one property is the same failure mode this PR exists to fix. A subscripted generic the generator doesn't model (tuple[int, str], set[int], Annotated[...]) still raises, as before.

Audit. __origin__/__args__ appear in exactly one function repo-wide, the one fixed here. Every other union check in the repo (plain-mcp's _type_to_schema, plain-postgres' written.py and preflight, plain's settings parser, plain-api's own typed_dict_from_annotation) already tests get_origin(t) in (Union, UnionType) with the union branch ahead of the generic branches, so nothing else is exposed. plain-mcp's version is the same function written correctly.

Eight tests, all confirmed failing against the pre-fix code, including one that generates a whole document with str | None, list[str | None] and str | int | None fields and validates it against the OpenAPI 3.0.3 schema — the regression at the level where the 500 appears.

Verified: ./scripts/fix clean; plain-api 53 passed; ./scripts/type-check plain-api and ./scripts/type-validate clean.

[pullapprove5]

PASS: 1 review scope passed

	Scope	Progress
✅	code	1/1

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• @davegaeddert: Merge or close