feat(lmxproxy): replace subscribe/unsubscribe health probe with persistent subscription

The old probe did a subscribe-read-unsubscribe cycle every 5 seconds to
check connection health. This created unnecessary churn and didn't detect
the failure mode where long-lived subscriptions silently stop receiving
COM callbacks (e.g. stalled STA message pump). The new approach keeps a
persistent subscription on the health check tag and forces reconnect if
no value update arrives within a configurable threshold (ProbeStaleThresholdMs,
default 5s). Also adds STA message pump debug logging (5-min heartbeat with
message counters) and fixes log file path resolution for Windows services.

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

lmxproxy/tests/ZB.MOM.WW.LmxProxy.Host.Tests/Subscriptions/SubscriptionManagerTests.cs

@@ -30,8 +30,6 @@ namespace ZB.MOM.WW.LmxProxy.Host.Tests.Subscriptions
                 IReadOnlyDictionary<string, object> values, string flagTag, object flagValue,
                 int timeoutMs, int pollIntervalMs, CancellationToken ct = default) =>
                 Task.FromResult((false, 0));
-            public Task<ProbeResult> ProbeConnectionAsync(string testTagAddress, int timeoutMs, CancellationToken ct = default) =>
-                Task.FromResult(ProbeResult.Healthy(Quality.Good, DateTime.UtcNow));
             public Task UnsubscribeByAddressAsync(IEnumerable<string> addresses) => Task.CompletedTask;
             public Task<IAsyncDisposable> SubscribeAsync(IEnumerable<string> addresses, Action<string, Vtq> callback, CancellationToken ct = default) =>
                 Task.FromResult<IAsyncDisposable>(new FakeSubscriptionHandle());