Add onError hook to middlewareEventConsumer

[thedevyashsaini]

Context middlewareEventConsumer currently logs validation / gRPC errors internally but does not allow callers to react. There are TODOs in Scrawn.js/packages/scrawn/src/core/scrawn.ts indicating a callback is desired.

Goal Allow SDK users to provide an optional onError callback that runs when the middleware fails to track an event (validation or gRPC failure).

Acceptance Criteria

• middlewareEventConsumer config accepts optional onError callback.
• onError receives { error, req, payload? } (or equivalent) for:
  • validation failure
  • gRPC failure
  • unexpected runtime error
• Existing behavior remains backward compatible.

Notes

• Keep callback optional
• Do not throw from middleware unless user callback throws.