Fix complete cancellation and termination diagnostics when a reason contains
a literal NUL. Terminal history and the failure row now store the same lossless
JSON string literal on every supported database. The original reason remains
unchanged in Avro command and history payloads.
Ordinary messages and SDK cancellation outcomes retain their behavior. No
schema migration is required.
Fix: #742
The issue tracks the pinned Server image update and published consumer
verification: #741