fix: stabilize test container startup

[disintegrator]

Summary

• Wait for the Docker daemon with bounded retries before starting PostgreSQL, Redis, or ClickHouse test containers.
• Coalesce concurrent Docker readiness calls and cache successful readiness for the test process.
• Require the ClickHouse container's final network listener before reporting readiness while retaining client-side ping retries.

Motivation

Blacksmith runners can begin a test while Docker's API is temporarily unresponsive. Testcontainers caches its first Docker host discovery result, so a transient docker info timeout otherwise prevents every container from starting in that test process. A singleflight preflight makes concurrent container constructors share one bounded readiness attempt and error; later calls can retry a failure, while success remains cached.

The ClickHouse image also starts a loopback-only bootstrap server while applying initialization scripts. Waiting through the container hostname prevents that temporary server from satisfying readiness while preserving the existing host-side retry protection.

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Latest commit: a3e50fc

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[gram-bot]

Auto-approved: review:bypass label applied by @disintegrator. Required status checks still gate this merge.