fix: support StringConstraints metadata in SQLAlchemy type generation

[Sujit-1509]

Summary

When using Pydantic v2's StringConstraints inside Annotated, SQLModel correctly applies validation constraints but does not propagate max_length to the generated SQLAlchemy column type.

For example:

from typing import Annotated
from pydantic import StringConstraints

class Item(SQLModel, table=True):
    id: int | None = Field(default=None, primary_key=True)
    name: Annotated[str, StringConstraints(max_length=50)]

currently generates a column with no length constraint, even though max_length=50 is declared.

Root Cause

get_field_metadata() recognizes PydanticMetadata and MaxLen metadata objects, but does not recognize StringConstraints, causing the metadata extractor to return FakeMetadata() instead of the constraint object.

As a result, get_sqlalchemy_type() cannot read max_length and falls back to an unconstrained string type.

Changes

• Add support for StringConstraints in get_field_metadata().
• Add a regression test verifying that Annotated[str, StringConstraints(max_length=50)] produces a SQLAlchemy column with length == 50.

Testing

Added a regression test that fails on the current implementation and passes with this change.

[YuriiMotov]

@Sujit-1509, do you have specific use case there it's required to use StringConstraints instead of passing max_length via Field?

You marked it as fix, but this is not a fix because it's not documented in SQLMOdel's docs.
To accept this as a feature request we need stronger arguments.
Please, avoid opening PRs just to contribute to the project. We are overloaded with PRs..