Python 3.14: unquoted forward references fail during Struct class creation #1165

Description

### Description

On Python 3.14, an unquoted forward reference in a `msgspec.Struct`
field raises `NameError` while the Struct class is being created.

Python 3.14's default deferred-annotation behavior normally permits this
without quoting. The equivalent example using ordinary classes works.

Tested with:

- CPython 3.14.0
- msgspec 0.21.1 (currently the latest release on PyPI)

### Minimal reproducer

```python
import msgspec


class Outer(msgspec.Struct):
    inner: Inner


class Inner(msgspec.Struct):
    value: int

Actual result

Traceback (most recent call last):
  File "<string>", line 6, in <module>
    class Outer(msgspec.Struct):
        inner: Inner
  File "<string>", line 7, in __annotate__
    inner: Inner
           ^^^^^
NameError: name 'Inner' is not defined

Expected result

I expected Outer to be created successfully, with Inner resolved after
the module finishes defining it, as happens with an ordinary Python 3.14
class:

class Outer:
    inner: Inner


class Inner:
    value: int


assert Outer.__annotations__ == {"inner": Inner}

Workarounds

Either of the following prevents the error:

class Outer(msgspec.Struct):
    inner: "Inner"

or:

from __future__ import annotations

Both workarounds use stringified annotations instead of Python 3.14's
default deferred-annotation behavior.

Additional context

This appears related to:

• Update annotation parsing to work with PEP 649 in Python 3.14 #651
• PR Fix annotations support on 3.14 #852

PR #852 added Python 3.14 annotation support, but Struct field collection
currently invokes the class annotate function with
annotationlib.Format.VALUE:

https://github.com/msgspec/msgspec/blob/main/src/msgspec/_core.c#L6110-L6133

Python's annotationlib documentation recommends that metaclasses
inspecting annotations during class construction generally use
Format.FORWARDREF, because names may not yet be resolvable:

https://docs.python.org/3.14/library/annotationlib.html#using-annotations-in-a-metaclass

Would using Format.FORWARDREF, followed by the existing delayed
forward-reference resolution, be appropriate here?

A practical consequence is that Ruff's UP037 rule considers the explicit
quotes unnecessary on Python 3.14, but removing them can make modules
containing msgspec.Struct schemas fail to import.