ci: keep request replayer responsive under parallel tests

[bm1549]

Description

High-level change and expected CI impact report

Stacked on #4097.

test_extension_ci runs as many as 12 PHPT workers against one single-threaded request-replayer. If its worker gets stuck processing a request, /replay also blocks. The test process then waits on agent requests until the job reaches its two-hour timeout.

Run request-replayer with 16 PHP CLI workers. Its file-backed queues use read-modify-write operations, so each session now has a file lock around state changes. MessagePack decoding stays outside the lock. Requests for different sessions can still run at the same time, including the un-tokened PHPT path.

The CI service gates HTTP requests until the checked-out router and lock helper are installed. The installer retries missing files and transient copy failures. UDP metrics runs under a restart loop.

This is separate from #4097: that PR splits normal and valgrind passes and raises test parallelism; this PR keeps request-replayer responsive under that parallel workload.

Verification

• Production-router regression test passes locally and in php:8.2-cli with 16 workers.
• Removing the state lock makes the regression test fail with lost concurrent requests.
• The exact CI image retained 200 concurrent requests and passed replay, metrics, stats, remote-config, agent-info, clear, and next-response checks.
• Early traffic remained gated during a simulated delayed checkout; after installation, the service retained 100 concurrent requests without restarting its listener.
• Killing the UDP metrics process produced a replacement process, which accepted and replayed a tagged metric.
• The test harness passed 12 consecutive runs in the exact generate-templates image with one CPU and 256 MiB. Child logs use files rather than undrained pipes and are printed if the test fails.
• PHP and shell syntax checks pass; git diff --check is clean.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[datadog-datadog-prod-us1-2]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🔄 Datadog auto-retried 1 job - 1 passed on retry

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 60.63% (+0.03%)

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