fix: preserve task and entity type safety

[K-Mistele]

Summary

• preserve exact state and event schema definitions through MachineEntity
• make .task() retain its state, event, error, and service types without any or diagnostic suppressions
• require shorthand tasks without onSuccess to return a valid machine event and guard that invariant at runtime
• add compile-time regressions and a patch changeset

Validation

• bun run gate
• 299 tests passing