fix(SEC-36): scrub committed dev LDAP service-account password; add user-secrets channel + rotation runbook

Repo-side half of SEC-36. The appsettings.json plaintext was already discharged
before this branch (HEAD ships the fail-closed ${secret:ldap/mxgateway/bind}
store reference), so the residual leak was the literal value in glauth.md,
docs/GatewayTesting.md, and the historical archreview SEC-06 evidence -- all
scrubbed to <service-account-password> placeholders pointing at the source of
truth scadaproj/infra/glauth/.

- csproj: add <UserSecretsId>mxaccessgw-server</UserSecretsId> (dev channel)
- GatewayOptionsValidator: blank-password message now names both channels
  (dev user-secrets, deployed MxGateway__Ldap__ServiceAccountPassword)
- test: assert the message names both channels
- docs: GatewayConfiguration.md (three channels + rotation note), glauth.md
  (placeholders + rotation-required + runbook pointer), GatewayTesting.md
- new operator runbook docs/runbooks/SEC-36-ldap-credential-rotation.md
  (live rotation + NSSM staging remain operator-pending)
- tracking: SEC-36 -> Done (repo-side) in both registers + change-log

Deviation: kept the ${secret:} reference in appsettings.json rather than
deleting it (spec step 2 assumed the stale plaintext baseline); deleting it
would regress the shipped/documented/tested secret-store channel.

git grep -i for the old value is empty across all tracked files.

src/ZB.MOM.WW.MxGateway.Server/Configuration/GatewayOptionsValidator.cs

@@ -145,7 +145,12 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
             builder);
         AddIfBlank(
             options.ServiceAccountPassword,
-            "MxGateway:Ldap:ServiceAccountPassword is required when LDAP login is enabled.",
+            "MxGateway:Ldap:ServiceAccountPassword is required when LDAP login is enabled. "
+                + "Never commit it: on dev boxes set user-secrets "
+                + "(dotnet user-secrets set \"MxGateway:Ldap:ServiceAccountPassword\" <value>); "
+                + "on deployed hosts set the environment variable "
+                + "MxGateway__Ldap__ServiceAccountPassword. "
+                + "(appsettings.json ships the ${secret:ldap/mxgateway/bind} store reference as the default.)",
             builder);
         AddIfBlank(
             options.UserNameAttribute,

src/ZB.MOM.WW.MxGateway.Server/ZB.MOM.WW.MxGateway.Server.csproj

@@ -2,6 +2,10 @@
 
   <PropertyGroup>
     <TargetFramework>net10.0</TargetFramework>
+    <!-- Dev-box channel for MxGateway:Ldap:ServiceAccountPassword (SEC-36): user-secrets
+         are loaded automatically in the Development environment and live under the user
+         profile, outside the tree, so the shared GLAuth bind credential is never committed. -->
+    <UserSecretsId>mxaccessgw-server</UserSecretsId>
   </PropertyGroup>
 
   <ItemGroup>