Fix ForwardRef resolution inside Annotated (Closes #13056)

Problem

When using from __future__ import annotations, type annotations are stored as strings (ForwardRef). FastAPI resolves forward references when the annotation itself is a string, but fails when the ForwardRef is wrapped inside typing.Annotated.

Example:

from __future__ import annotations
from typing import Annotated
from fastapi import Depends

@app.get("/")
async def read_root(potato: Annotated["Potato", Depends(get_potato)]):
    ...

In this case, "Potato" is not resolved correctly, which leads to incorrect dependency parsing and incorrect OpenAPI schema generation.

Root Cause

get_typed_annotation() only resolves forward references when the annotation is directly a str or ForwardRef. It does not handle ForwardRefs nested inside Annotated.

Solution

• Detect Annotated types using get_origin
• Extract base type and metadata via get_args
• Resolve ForwardRef for the base type (handles both str and ForwardRef)
• Recursively resolve nested annotations
• Reconstruct Annotated using __class_getitem__ to preserve correct structure

Changes

• Added ForwardRef resolution for base types inside Annotated
• Added recursive handling for nested Annotated cases
• Preserved existing behavior for non-Annotated types
• Ensured consistent handling of NoneType

Tests

Added test to validate:

• Annotated[Potato, Depends(...)] (existing behavior unchanged)
• Annotated["Potato", Depends(...)] (ForwardRef correctly resolved)
• OpenAPI schema generation does not leak unresolved ForwardRef strings

Notes

• Change is minimal and localized to get_typed_annotation()
• No breaking changes introduced
• Compatible with existing type resolution flow

Closes #13056