Trigger protobuf conversion from build artifact upload

[lucy66hw]

Summary

• trigger opensearch-protobufs from build.yml immediately after uploading the merged spec artifact
• pass the uploaded workflow artifact URL in the dispatch payload
• remove the old protobuf trigger from deploy-gh-pages.yml

Why this change is needed

The protobuf conversion flow should not depend on main-latest release publication succeeding. By dispatching right after artifact upload, downstream protobuf generation can consume the merged spec artifact even if the later GitHub release step fails.

Behavior change

• before: protobuf conversion was triggered from deploy-gh-pages.yml and expected main-latest
• now: protobuf conversion is triggered from build.yml after artifact upload and receives artifact_url