fix: ensure RequestHandler type is accepted in setup functions

[FelmonFekadu]

Summary

• accept base RequestHandler instances in the shared AnyHandler type
• keep setupWorker and setupServer compatible with explicitly typed RequestHandler[] arrays
• add regression coverage for both setup APIs

Verification

• pnpm build
• pnpm test:ts
• pnpm test:unit -- --run src/core/experimental/handlers-controller.test.ts test/node/msw-api/setup-server/listHandlers.node.test.ts
• pnpm lint

Fixes the compatibility gap reported while testing #2650, where arrays typed as RequestHandler[] no longer spread into setupWorker/setupServer.

[kettanaito]

Thanks for raising this, @FelmonFekadu. I've improved the type tests to actually have assertions and cleaned up the manual AnyHandler type from the core. Looks good now.

[FelmonFekadu]

Thanks for tightening that up and taking it the rest of the way. I appreciate the cleanup and the stronger type assertions.