Fix Valkey transport degradation: command timeouts, health checks, cleanup

Root cause of 504 gateway timeouts after ~20 min of continuous use:
1. No Redis command-level timeout — StackExchange.Redis commands hung
   indefinitely when Valkey was slow, creating zombie connections
2. IsConnected check missed zombie connections — socket open but unable
   to execute commands, so all requests reused the hung connection
3. Slow cleanup — expired pending requests cleaned every 30s, accumulating
   faster than cleanup could remove them under sustained load

Fixes:
- ValkeyConnectionFactory: Add SyncTimeout=15s and AsyncTimeout=15s to
  ConfigurationOptions. Commands now fail fast instead of hanging.
- ValkeyConnectionFactory: Add PING health check in GetConnectionAsync().
  If PING fails, connection is considered zombie and reconnected.
- CorrelationTracker: Reduce cleanup interval from 30s to 5s. Expired
  pending requests are now cleaned 6x faster, preventing dictionary bloat.

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

src/Router/__Libraries/StellaOps.Messaging.Transport.Valkey/ValkeyConnectionFactory.cs

@@ -44,7 +44,18 @@ public sealed class ValkeyConnectionFactory : IAsyncDisposable
     {
         if (_connection is not null && _connection.IsConnected)
         {
-            return _connection;
+            // Verify the connection can actually execute commands (not zombie).
+            // IsConnected only checks the socket, not command execution ability.
+            try
+            {
+                await _connection.GetDatabase().PingAsync().ConfigureAwait(false);
+                return _connection;
+            }
+            catch
+            {
+                _logger?.LogWarning("Valkey connection PING failed — reconnecting");
+                // Fall through to reconnect
+            }
         }
 
         await _connectionLock.WaitAsync(cancellationToken).ConfigureAwait(false);
@@ -61,6 +72,8 @@ public sealed class ValkeyConnectionFactory : IAsyncDisposable
                 var config = ConfigurationOptions.Parse(_options.ConnectionString);
                 config.AbortOnConnectFail = _options.AbortOnConnectFail;
                 config.ConnectTimeout = (int)_options.InitializationTimeout.TotalMilliseconds;
+                config.SyncTimeout = 15_000;   // 15s — prevents commands from hanging indefinitely
+                config.AsyncTimeout = 15_000;  // 15s — async command timeout
                 config.ConnectRetry = _options.ConnectRetry;
 
                 if (_options.Database.HasValue)

src/Router/__Libraries/StellaOps.Router.Transport.Messaging/Protocol/CorrelationTracker.cs

@@ -20,8 +20,9 @@ public sealed class CorrelationTracker : IDisposable
     public CorrelationTracker(TimeProvider? timeProvider = null)
     {
         _timeProvider = timeProvider ?? TimeProvider.System;
-        // Cleanup expired requests every 30 seconds
-        _cleanupTimer = _timeProvider.CreateTimer(CleanupExpiredRequests, null, TimeSpan.FromSeconds(30), TimeSpan.FromSeconds(30));
+        // Cleanup expired requests every 5 seconds (was 30s — too slow under sustained load,
+        // causing pending request dictionary to grow and degrade lookup performance)
+        _cleanupTimer = _timeProvider.CreateTimer(CleanupExpiredRequests, null, TimeSpan.FromSeconds(5), TimeSpan.FromSeconds(5));
     }
 
     /// <summary>