fix: Make map.Map and maptree.MapTree require keys to be hashable

[jonathandung]

Seeing as though the map.Map class implements the Mapping interface, the type variable used to annotate its keys should require hashability, and the same goes for maptree.MapTree, used internally by the above only.

[jonathandung]

Parent: python/typeshed#15754

[dbrattli]

Thanks for the typeshed-alignment proposal. Before this can be reviewed for merge, please rebase onto the current main (the branch is nine commits behind) and add focused type-check coverage demonstrating that comparable-but-unhashable keys are rejected while standard ordered, hashable keys remain accepted. Please also simplify the new protocol implementation to a declaration-only member (...) rather than calling super().__hash__(); the protocol is structural and has no meaningful runtime implementation.

[dbrattli]

Approved. The hashable key bound correctly aligns Map with the Mapping contract and its existing use of key hashing. My previous request reflected integration preferences rather than a design blocker; a maintainer update/rebase can be handled as part of merge preparation.

[dbrattli]

Fixed the strict Pyright failures in abf8370 by restoring MapTree.empty and size to key-agnostic internal annotations. The public Map/MapTree key bound remains comparable-and-hashable; Pyright, Map tests, and Ruff pass.