test(scriptworker): tight-heartbeat tests warm the worker before arming the 3-interval silence kill (#827 audit)

A_burst_of_cold_compiles_does_not_starve_the_heartbeat (50 ms heartbeat) and
A_caller_that_blocks_after_its_result_does_not_starve_the_reader (100 ms)
launched their worker under the tight silence kill, so its cold start (launch,
first compile, Roslyn JIT) had to stay within 150/300 ms of silence. Under 28
busy processes on 14 cores that failed 2 in 10 runs in the WARM-UP, killed
for ~165 ms of silence, and replacement workers were killed the same way.

New test seam: WorkerLaunchSpec.HeartbeatSilenceLimit (runner internal ctor
heartbeatSilenceLimit) and WorkerConnection.SilenceLimit. The tests launch
with a 30 s limit, warm, then tighten to three intervals for the phase they
are about; the assertions on that phase are unchanged.

src/Core/ZB.MOM.WW.OtOpcUa.Core.Scripting.OutOfProcess/WorkerConnection.cs

@@ -81,6 +81,10 @@ internal sealed record BatchSubmission(string Key, EvalRequest Request, TimeSpan
 ///     Windows only: the worker identity's SID (<c>Scripting:Worker:Identity</c>). Set, the worker starts under a restricted
 ///     token (<see cref="RestrictedWorkerLauncher"/>) and the pipe ACL admits it beside the host; null keeps the host's token.
 /// </param>
+/// <param name="HeartbeatSilenceLimit">
+///     How long the worker may send nothing before it is killed (<see cref="WorkerExitReason.KilledHeartbeat"/>); null =
+///     three <paramref name="HeartbeatInterval"/>s. Test seam (with <see cref="WorkerConnection.SilenceLimit"/>).
+/// </param>
 /// <param name="WorkerSoftClock">
 ///     Ask the worker to run the soft clock from each run's start (lmxopcua#827; see <see cref="WorkerConnection.WorkerSoftClock"/>).
 ///     False (tests only) keeps the pre-#827 host clock, which is also what a worker that does not echo it gets.
@@ -99,7 +103,8 @@ internal sealed record WorkerLaunchSpec(
     TimeSpan? LoadTimeout = null,
     int CompileThreads = 0,
     string? WorkerSid = null,
-    bool WorkerSoftClock = true)
+    bool WorkerSoftClock = true,
+    TimeSpan? HeartbeatSilenceLimit = null)
 {
     /// <summary>The default compile timeout.</summary>
     public static readonly TimeSpan DefaultLoadTimeout = TimeSpan.FromSeconds(60);
@@ -156,7 +161,7 @@ internal sealed class WorkerConnection : IAsyncDisposable
 
     private WorkerConnection(string shard, Process process, NamedPipeServerStream pipe, IDisposable? job, IDisposable? stdio,
         string workDir, ILogger log, Action<LogBatch> onLogs, TimeSpan heartbeatInterval, TimeSpan loadTimeout,
-        Action<WorkerExit> onExiting, string? markerPath, bool workerSoftClock)
+        Action<WorkerExit> onExiting, string? markerPath, bool workerSoftClock, TimeSpan? heartbeatSilenceLimit)
     {
         WorkerSoftClock = workerSoftClock;
         _stdio = stdio;
@@ -171,6 +176,7 @@ internal sealed class WorkerConnection : IAsyncDisposable
         _log = log;
         _onLogs = onLogs;
         _heartbeatInterval = heartbeatInterval;
+        _silenceLimitTicks = (heartbeatSilenceLimit ?? heartbeatInterval * 3).Ticks;
         Pid = process.Id;
         // The handshake used seq 1. A failed write means the pipe is gone: the worker is dead or dying.
         _writer = new ScriptWorkerFrameWriter(pipe, OnWriteFault, ScriptWorkerFrameCodec.DefaultMaxFrameBytes, firstSeq: 1,
@@ -215,6 +221,20 @@ internal sealed class WorkerConnection : IAsyncDisposable
         set => _loadTimeout = value;
     }
 
+    /// <summary>
+    ///     How long the worker may send nothing before it is killed for silence: three heartbeat intervals unless the
+    ///     launch spec said otherwise. Test hook: a test with a tight heartbeat launches and warms the worker (its cold
+    ///     start JITs Roslyn, which on a loaded box can pause it for longer than three short intervals) under a generous
+    ///     limit, then tightens it for the phase it is about.
+    /// </summary>
+    internal TimeSpan SilenceLimit
+    {
+        get => TimeSpan.FromTicks(Volatile.Read(ref _silenceLimitTicks));
+        set => Volatile.Write(ref _silenceLimitTicks, value.Ticks);
+    }
+
+    private long _silenceLimitTicks;
+
     /// <summary>Compiles currently outstanding on this worker.</summary>
     public int LoadingCount => _loads.Count;
 
@@ -327,7 +347,8 @@ internal sealed class WorkerConnection : IAsyncDisposable
                 : await HandshakeAsync(pipe, hello, nonce, process.Id, handshakeCts.Token).ConfigureAwait(false);
 
             var connection = new WorkerConnection(spec.Shard, process, pipe, job, stdio, workDir, log, onLogs, spec.HeartbeatInterval,
-                spec.EffectiveLoadTimeout, onExiting, markerPath, workerSoftClock: spec.WorkerSoftClock && ready.WorkerSoftClock);
+                spec.EffectiveLoadTimeout, onExiting, markerPath, workerSoftClock: spec.WorkerSoftClock && ready.WorkerSoftClock,
+                heartbeatSilenceLimit: spec.HeartbeatSilenceLimit);
             connection.Begin();
             return connection;
         }
@@ -888,7 +909,7 @@ internal sealed class WorkerConnection : IAsyncDisposable
             {
                 if (_exited.Task.IsCompleted) return;
                 var silent = Stopwatch.GetElapsedTime(Volatile.Read(ref _lastFrameTimestamp));
-                if (silent > _heartbeatInterval * 3)
+                if (silent > SilenceLimit)
                 {
                     _log.Warning("ScriptWorker[{Shard}] pid {Pid} sent nothing for {Silent} ms; killing it", Shard, Pid, (long)silent.TotalMilliseconds);
                     Kill(WorkerExitReason.KilledHeartbeat, null);