perf(runtime): O(1) attribute resolution, precomputed types, coalesced static writes, shared JSON options

src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs

@@ -27,6 +27,15 @@ public class ExternalSystemClient : IExternalSystemClient
     private readonly ILogger<ExternalSystemClient> _logger;
     private readonly ExternalSystemGatewayOptions _options;
 
+    /// <summary>
+    /// Shared options for the per-call AuthConfiguration JSON parse in
+    /// <see cref="TryParseJsonAuth"/> (perf remediation, arch-review WP1.5) — avoids
+    /// allocating a new <see cref="JsonSerializerOptions"/> (and its internal cached
+    /// metadata) on every outbound call. Settings preserved exactly.
+    /// </summary>
+    private static readonly JsonSerializerOptions AuthJsonOptions =
+        new() { PropertyNameCaseInsensitive = true };
+
     /// <summary>
     /// Initializes a new instance of the ExternalSystemClient.
     /// </summary>
@@ -755,7 +764,7 @@ public class ExternalSystemClient : IExternalSystemClient
         {
             fields = JsonSerializer.Deserialize<Dictionary<string, string?>>(
                 config,
-                new JsonSerializerOptions { PropertyNameCaseInsensitive = true })
+                AuthJsonOptions)
                 ?? new Dictionary<string, string?>(StringComparer.OrdinalIgnoreCase);
 
             // Normalize to case-insensitive so "Header"/"header" both resolve.