Reparent 8 string constraints under PatternConstraint

Eliminates duplicated validate() and __get_pydantic_json_schema__()
across CountryCodeAlpha2, HexColor, LanguageTag, NoWhitespace,
SnakeCase, PhoneNumber, RegionCode, and WikidataId constraints.
Each is now a thin __init__-only wrapper calling super().__init__().

PatternConstraint gains optional keyword-only description, min_length,
max_length parameters for JSON Schema annotations. StringConstraint
gains _raise_validation_error() to deduplicate error construction
across PatternConstraint, JsonPointerConstraint, and
StrippedConstraint.

Author: sethfitz