Concurrent createDatabase requests for the same database can join an existing coordinator. A mongosync request with write-block bypass enabled may therefore join a coordinator created by a regular user without bypass and inherit its UserWritesBlocked result.

Update the joinability rule:

• When user sharded-DDL blocking is disabled, requests may join regardless of bypass metadata.
• When blocking is enabled, bypass metadata must match.
• A mismatch returns ConflictingOperationInProgress, allowing mongosync to retry.