Support constraints on Optional (union with None) types

[Siyet]

Summary

• Derive constraint_kind from non-None union members so that Annotated[X | None, Meta(...)] constraints are validated correctly
• Previously any constraint on an Optional[X] type raised TypeError because the union branch left kind as CK_OTHER
• Supports all constraint kinds: numeric (gt/ge/lt/le/multiple_of), string (min_length/max_length/pattern), bytes, collections, datetime/time (tz)

Closes #804

Test plan

All 275 constraint tests pass (Ubuntu 22.04, Python 3.10):

tests/unit/test_constraints.py - 275 passed in 0.27s

New tests added:

• test_optional_str_min_length - Annotated[Union[str, None], Meta(min_length=3)]
• test_optional_int_gt - Annotated[Union[int, None], Meta(gt=0)]
• test_optional_float_ge - Annotated[Union[float, None], Meta(ge=0.0)]

Each test verifies:

• None values pass through without constraint checks

• Valid values are accepted

• Invalid values raise ValidationError

• CI passes on all platforms

[Siyet]

CI failures are unrelated to this change:

"Profile Windows (ARM64)" - runner infra issue ("Install command runner" step)
"build" (documentation) - link checker fails on a pre-existing broken link (#976)
All build, test, and wheel jobs pass across all platforms.

[Siyet]

Resolved in the community fork msgspec-arise: PR #10, released in v0.20.2.

[jcrist]

Not supporting this was intentional.

We only support applying annotations directly to the type they apply to for clarity.

# This works, and makes it clear that the annotation applies to the `int`
Annotated[int, Meta(ge=1)] | None

# This doesn't work, but IMO it shouldn't. None isn't numeric and can't support >=
Annotated[int | None, Meta(ge=1)]

Since Optional[int] is the same as int | None, it's a slippery slope to applying annotations to a union and expecting them to apply to all union members. Annotated[int | str | None, Meta(ge=1)] for example. I'd rather just keep things simple and clear - annotations must be applied to the type directly, and not the union.