fix: do not invent Unknown error on successful executions

[Pitchfork-and-Torch]

Bug

n8n_executions with mode=error on an execution whose top-level status is success still built an error analysis. extractPrimaryError fell back to the last node and the string Unknown error, and buildExecutionPath marked that node as failed. Maintainers hit this live on n8n 2.37.7 (issue #1065).

Fix

If there is no resultData.error and no node-level error, error mode now returns success: true, a None primary error that says there is nothing to diagnose, and an execution path that keeps real node statuses.

Tests

Unit test covers a successful three-node execution: no invented error, no error status on the path, Nothing to diagnose suggestion.

Closes #1065

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/services/error-execution-processor.ts	90.90%	4 Missing and 1 partial ⚠️

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