OAS 3.1 multi-type cast/deserialize support

[p1c2u]

Relates #1180

• openapi_core/schema/types.py (new) pick_style_type(): maps str | list[str] → structural type (array > object > primitive).
• casting/schemas/casters.py added MultiTypeCaster: iterates declared types, returns first successful cast, raises one CastError with the full type list on miss.
• casting/schemas/init.py wired multi=MultiTypeCaster for OAS 3.1/3.2; OAS 3.0 stays multi=None.
• deserializing/styles/util.py every loader (simple, form, matrix, label, delimited, deepObject) routes schema_type through pick_style_type.
• deserializing/media_types/deserializers.py multipart array fan-out uses pick_style_type + read_str_or_list.
• deserializing/styles/casters.py deleted (dead code).