feat(stovepipe): Announce validation results

[prathshenoy]

Summary

What:

• Publish an event whenever a commit's validation result becomes durable, carrying whether the commit is healthy or broken and what it was measured against.
• Publish a distinct event when validation ends without a verdict, and fail the work when either event cannot be sent.

Why:

• Let subscriber systems act on trunk validation results as they land, instead of polling for them.
• Keep the event vendor-neutral so each deployment wires its own subscriber to forward results onward, instead of building any single environment's delivery path into the pipeline.

Test Plan

• Add unit tests.

Revert Plan

• Revert this PR. The events are additive and nothing consumes them yet.

Issues

• CODEM-459