Covers integrations/salesforce.service.ts: disabled-mode logging-only
path, signup-not-found, success (token fetch + POST + mark SENT), token
reuse across calls, retry-once-on-401, failure marks FAILED with the
error message, and a missing-client-credentials configuration error.
100% statement coverage on the file.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
