fix(test): skip the Redis container tests when Docker is unreachable (#526)

[JabbaKadabra]

Summary

RedisIngestionStreamIntegrationTests is meant to report as skipped when no container runtime is reachable, but both tests failed on a Docker-less machine, making a plain dotnet test Proxytrace.sln red for reasons unrelated to the change under test. The container was built one line above the try, and Testcontainers validates a builder by resolving and pinging the Docker endpoint — so new RedisBuilder(RedisImage).Build() is where the throw actually happens and the catch (Exception ex) when (!DockerRequired) guard never saw it. This moves Build() inside the guarded block so endpoint-resolution failures take the same skip path as start failures.

Closes #526

Changes

• Proxytrace.Messaging.Tests/RedisIngestionStreamIntegrationTests.cs — construct the RedisContainer inside the guarded try, hold it in a nullable local, and null-check before disposing on the skip path. No ! suppression needed: Assert.Inconclusive is [DoesNotReturn], so the non-null return flows cleanly.
• docs/testing.md — the container-backed section now states that the guard must wrap Build(), not just StartAsync, and why.
• .claude/skills/test/SKILL.md — same one-line rule in the container-backed-tests section, so the next such suite does not re-derive it.

Verification

• build: dotnet build Proxytrace.Messaging.Tests -warnaserror — 0 warnings, 0 errors.
• tests (no Docker socket present in this environment, which is the exact repro):
  • dotnet test Proxytrace.Messaging.Tests — Failed: 0, Passed: 11, Skipped: 2 (was Failed: 2 before the change).
  • PROXYTRACE_REQUIRE_DOCKER_TESTS=1 dotnet test Proxytrace.Messaging.Tests --filter "FullyQualifiedName~RedisIngestionStreamIntegrationTests" — Failed: 2, confirming the enforce path still turns a missing runtime into a hard failure, so CI cannot lose the coverage silently.
  • Scope: the affected test project only. The change touches one test file plus docs; the rest of the suite is unaffected and CI runs the full solution on push.
• docs/changelog: docs/testing.md and the test skill updated. No CHANGELOG.md entry — the changelog is for user-facing changes and this is test-harness-only, with no product behaviour affected.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RVMGTQ2k3CwrkcannhZ9sA

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