n8n_executions mode=error reports "Unknown error" on a successful execution

Observed while live-testing v2.80.1 on n8n 2.37.7 (behaviour predates that release; the same code is in v2.79.1).

Calling n8n_executions with action=get, mode=error on an execution whose top-level status is success returns an analysis with primaryError: { message: "Unknown error", nodeName: <last node> } and an executionPath that marks that node as status: "error", because the error processor picks the last node as the error node when the execution carries no error and always renders that node as failed.

Expected: when the execution has no error, error mode should say so (for example a success: true analysis with a note that there is nothing to diagnose, or fall back to summary mode) instead of inventing an error.

Where: src/services/error-execution-processor.ts, extractPrimaryError (falls back to lastNode and 'Unknown error') and buildExecutionPath (forces the primary error node to status: 'error').