test: speed up test suite

[djwhitt]

Summary

• replace fixed sleeps and incidental timing with process monitors, messages, state synchronization, and retry assertions
• reduce expensive test setup by reusing valid event fixtures and enqueueing backend logs without starting unrelated source supervision
• shorten producer and Broadway batching latency only in the test environment
• make ClickHouse error paths, provisioner checks, and hostname parsing tests deterministic
• preserve integration boundaries, queue assertions, timer/lifecycle behavior, shorthand timestamp cases, and backend-specific telemetry coverage

Hosted CI results

Compared with the immediately preceding main run #7488, the passing PR run #7492 produced these results:

Metric	main	This PR	Improvement
ExUnit suite	9m 6.1s	4m 47.6s	47% faster
Synchronous phase	8m 36.9s	4m 20.2s	50% faster
Full test/coverage command	13m 29s	9m 4s	33% faster
Entire test job	15m 28s	10m 50s	30% faster

The PR run completed 4,512 tests, 49 properties, and 125 doctests with no failures. It ran one more test than the main baseline while reducing the ExUnit phase by 4m 18.5s and the complete test job by 4m 38s.