fix: enforce mediator handler contracts

[aiagate]

Summary

• Move Injector and handler registry to Mediator instances.
• Validate handler contracts, reject duplicates, and support abstract/intermediate generic handlers safely.
• Ship py.typed and add regression tests including isolated wheel type checking.

Verification

• pytest: 17 passed, 98.10% coverage
• Pyright: 0 errors
• Ruff: clean
• wheel/sdist build and examples: passed