Fix dead assertion in Slack private-channel audience test - #2748

Code Review Summary

PR #2748 replaces a dead assertion in the Slack private-channel audience test with an assertion against the mocked ensureCaptureAudience call. This aligns the test with the actual mock boundary and verifies the filtered member email, private-channel audience kind, and upstream reference hash that createSlackThreadCapture should pass through.

The approach is sound for this low-risk, test-only change. Two independent reviews found no confirmed correctness, security, or performance issues. The new assertion is materially stronger than reading mocks.rows.audienceMembers, which the fully mocked audience implementation does not populate, and the reported targeted and full-suite verification supports the change. No comments are required.

Risk assessment: Low risk (test-only change).

🧪 Browser testing: Skipped — PR only modifies backend/config/docs, no UI impact