COMPACTION_FAILURE_PREFIX

Stable prefix for forced-compaction failure tool messages.

/offload drives the tool server-side and can only observe the resulting ToolMessage text across the LangGraph server boundary, so it keys failure detection on this prefix. Owning the literal here means the producer (_forced_compact_error) and both consumers (app._drive_server_side_compaction live-stream detection and app._find_compaction_failure committed-state scan) reference one constant instead of re-hardcoding the wording independently.

Note: this value is deliberately identical to the leading text of the SDK's own model-initiated compaction-failure message, so a failure emitted by either path is recognized. Because the scan is bounded to messages produced by the current /offload attempt, a stale failure from an unrelated prior turn is not matched. Only the prefix position is load-bearing; wording after it is free to change.