feat(options): eager startup validation for edge/management options (InboundAPI, ESG, Notification, ManagementService) (arch-review 08 §1.5)

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

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

@@ -0,0 +1,33 @@
+using ZB.MOM.WW.Configuration;
+
+namespace ZB.MOM.WW.ScadaBridge.ExternalSystemGateway;
+
+/// <summary>
+/// Validates <see cref="ExternalSystemGatewayOptions"/> at startup.
+/// <see cref="ExternalSystemGatewayOptions.DefaultHttpTimeout"/> bounds each outbound call,
+/// <see cref="ExternalSystemGatewayOptions.MaxConcurrentConnectionsPerSystem"/> is fed into
+/// <c>SocketsHttpHandler.MaxConnectionsPerServer</c> (a non-positive value throws in the handler
+/// constructor), and <see cref="ExternalSystemGatewayOptions.MaxResponseBodyBytes"/> caps buffered
+/// response bodies. Registered with <c>ValidateOnStart()</c> so a bad
+/// <c>ScadaBridge:ExternalSystemGateway</c> section fails fast at boot with a clear, key-naming
+/// message rather than crashing the HTTP client factory or silently disabling the gateway.
+/// </summary>
+public sealed class ExternalSystemGatewayOptionsValidator : OptionsValidatorBase<ExternalSystemGatewayOptions>
+{
+    /// <inheritdoc />
+    protected override void Validate(ValidationBuilder builder, ExternalSystemGatewayOptions options)
+    {
+        builder.RequireThat(options.DefaultHttpTimeout > TimeSpan.Zero,
+            $"ScadaBridge:ExternalSystemGateway:DefaultHttpTimeout must be a positive duration " +
+            $"(was {options.DefaultHttpTimeout}); it bounds each outbound external-system HTTP call.");
+
+        builder.RequireThat(options.MaxConcurrentConnectionsPerSystem > 0,
+            $"ScadaBridge:ExternalSystemGateway:MaxConcurrentConnectionsPerSystem must be greater than zero " +
+            $"(was {options.MaxConcurrentConnectionsPerSystem}); it is fed into " +
+            "SocketsHttpHandler.MaxConnectionsPerServer, which rejects non-positive values.");
+
+        builder.RequireThat(options.MaxResponseBodyBytes > 0,
+            $"ScadaBridge:ExternalSystemGateway:MaxResponseBodyBytes must be greater than zero " +
+            $"(was {options.MaxResponseBodyBytes}); it caps the outbound response body buffered into memory.");
+    }
+}