Follow-up to the salvaged #111938 commit: `_slack_response_payload` already normalizes a
SlackResponse/dict body, so the new `_slack_api_error_code` helper and the two-branch
logger.error were redundant. One log line now always carries `api_error=<code|none>` so an
HTTP 200 + ok=false failure (e.g. message_not_found) is readable without exc_info.
Tests trimmed to one invariant per fix (session key on the response-ready line; API error
code on the edit failure); the `session=unknown` fallback test was a change-detector.
fb975fb098