feat(stovepipe): Announce validation start events

[prathshenoy]

Summary

What:

• Announce that validation of a commit has begun as soon as the pipeline admits it, alongside the existing announcements for validation outcomes.
• Repeat that announcement when an already-admitted request is redelivered, reusing the original event identity so consumers can discard the duplicate.

Why:

• Let downstream systems track a validation from start to finish, instead of only learning it happened once a verdict exists.
• Keep the announcement from being lost when the pipeline fails after durably admitting the request.

Test Plan

• Add unit tests.

Revert Plan

• Revert this PR. Nothing consumes the start event yet, so publishing just stops with no downstream impact.

Issues

• CODEM-466