Resolve 6 of 7 stability review findings and close test coverage gaps

Fixes P1 StaComThread hang (crash-path faulting via WorkItem queue), P1 subscription
fire-and-forget (block+log or ContinueWith on 5 call sites), P2 continuation point
leak (PurgeExpired on Retrieve/Release), P2 dashboard bind failure (localhost prefix,
bool Start), P3 background loop double-start (task handles + join on stop in 3 files),
and P3 config logging exposure (SqlConnectionStringBuilder password masking). Adds
FakeMxAccessClient fault injection and 12 new tests. Documents required runtime
assemblies in ServiceHosting.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/ZB.MOM.WW.LmxOpcUa.Tests/MxAccess/StaComThreadTests.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Concurrent;
+using System.Runtime.InteropServices;
 using System.Threading;
 using System.Threading.Tasks;
 using Shouldly;
@@ -13,6 +14,9 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.MxAccess
     /// </summary>
     public class StaComThreadTests : IDisposable
     {
+        [DllImport("user32.dll")]
+        private static extern void PostQuitMessage(int nExitCode);
+
         private readonly StaComThread _thread;
 
         /// <summary>
@@ -101,5 +105,20 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.MxAccess
 
             results.Count.ShouldBe(3);
         }
+
+        /// <summary>
+        ///     Confirms that after the message pump exits, subsequent RunAsync calls throw instead of hanging.
+        /// </summary>
+        [Fact]
+        public async Task RunAsync_AfterPumpExit_ThrowsInsteadOfHanging()
+        {
+            // Kill the pump from inside by posting WM_QUIT
+            await _thread.RunAsync(() => PostQuitMessage(0));
+            await Task.Delay(100); // let pump exit
+
+            _thread.IsRunning.ShouldBe(false);
+            Should.Throw<InvalidOperationException>(() =>
+                _thread.RunAsync(() => { }).GetAwaiter().GetResult());
+        }
     }
 }