Add workflow exist check to centralized registry workflow deploy

[j-nowak]

Summary

• Adds a pre-deploy workflow existence check so workflow deploy can detect existing workflows before upload and prompt for overwrite confirmation.
• Implements private-registry existence checks via GraphQL lookup by workflow name, and threads the existing workflow status through deploy flow.
• Refactors registry deploy strategies to share a CheckWorkflowExists path across onchain and private targets.
• Updates private-registry client models/operations to the offchain workflow API naming and adds coverage for success, not-found, and transport-error paths.

Testing

• Extended deploy and private-registry unit tests around existence checks, overwrite behavior, and GraphQL failure handling.
• Updated multi-command private-registry flow test to validate the new lookup + upsert operation sequence.

[timothyF95]

LGTM