Add RegisterTriggerWithHeaders to agent Runner for header-aware triggers

[harshitk-11]

The current RegisterTrigger API passes only m.Data (raw bytes) to the user-supplied callback, discarding the NATS message headers. This makes it impossible to access per-message metadata; auth context, tracing IDs, tenant identifiers, etc. - inside a trigger function.

Proposed Change

Add a companion method RegisterTriggerWithHeaders on Runner that passes the full *nats.Msg to the callback instead of just the payload bytes

• The reply-message path already attaches workload_id and namespace headers, so the plumbing for header-aware messaging is already present.
• RegisterTrigger could be reimplemented as a thin wrapper over RegisterTriggerWithHeaders to avoid duplicating the subscribe/publish/event-emit body.

Happy to do that in the PR if preferred.

[jordan-rash]

Thanks for the feedback @harshitk-11. This is a good callout. Ill open a PR for this in the next few days, there are some other things I might want to add to the callback

[harshitk-11]

Thanks for responding back @jordan-rash, that sounds good. Thanks very much