refactor: consolidate media constraint classes via generic bases

[Kamilbenkirane]

The six media constraint classes in constraints.py (ImageConstraint, ImagesConstraint, VideoConstraint, VideosConstraint, AudioConstraint, AudiosConstraint) contain nearly identical validation logic (~186 lines), differing only in artifact type, MIME enum, and label string.

Proposal: Introduce two private base classes (_MediaConstraint, _MediaListConstraint) to centralize the shared validation. Each public class becomes a thin subclass with typed __call__ signatures preserved for get_constraint_input_type introspection compatibility.

• Zero public API changes (same class names, constructors, error messages)
• All existing tests pass unchanged
• Net reduction of ~80 lines
• Adding a new media type becomes 2 thin subclasses instead of 2 copy-paste blocks