Fix pyyaml converter crashing on non-enum str subclasses

[Debasishhh]

The PyYAML preconf converter's str unstructure hook assumes any str subclass is a string enum and returns v.value. Any other subclass crashes:

from attrs import define
from markupsafe import Markup
from cattrs.preconf.pyyaml import make_converter

@define
class Page:
    title: str

make_converter().dumps(Page(Markup("<b>x</b>")))
# AttributeError: 'Markup' object has no attribute 'value'

Returning the value unchanged isn't enough, because yaml.safe_dump refuses str subclasses too. So the hook now keeps the enum path and turns any other subclass into a plain str. I moved it into a small named function since the one-line conditional was getting hard to read.

Added a test with a plain class MyStr(str) that checks the unstructured value is an exact str and that dumps/loads round-trips. The full test suite passes locally, and ruff check/ruff format are clean.