place null last in anyOf for optional unions in JSON schema

[SarthakB11]

Fixes #1027.

For T | None, _json_schema.py was emitting {"type": "null"} before
the struct/$ref entry in anyOf. Tools that pick the first anyOf
element for example generation (Swagger UI in the issue) ended up showing
null. Pydantic puts null last for the same reason.

Move NoneType out of the union iteration into a has_none flag and
append {"type": "null"} last in all three union branches (no-struct,
single-struct, tagged-struct). The tagged-struct branch's if options:
guard becomes if options or has_none: so Tagged | None keeps null
in its anyOf instead of dropping it.

Observable changes:

• Struct | None: was [null, $ref], now [$ref, null].
• Union[X, None, Y]: null moves from the middle to the end.
• str | None: already [string, null] on CPython, unchanged.

Tests added in tests/unit/test_schema.py for Optional[Struct],
Union[int, None, str], and Union[Tagged, None] (covers the third
branch that previously dropped null entirely).