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.Tests/Configuration/GatewayOptionsValidatorTests.cs

@@ -757,9 +757,15 @@ public sealed class GatewayOptionsValidatorTests
             new LdapOptions { Enabled = true, ServiceAccountPassword = string.Empty });
         ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
         Assert.True(result.Failed);
-        Assert.Contains(
+        string failure = Assert.Single(
             result.Failures!,
             f => f.Contains("MxGateway:Ldap:ServiceAccountPassword is required when LDAP login is enabled."));
+
+        // SEC-36: the message must steer the operator to the two supported out-of-band channels
+        // (dev user-secrets, deployed env var) so a blanked/unresolved credential never gets
+        // "fixed" by re-committing a value.
+        Assert.Contains("dotnet user-secrets set", failure);
+        Assert.Contains("MxGateway__Ldap__ServiceAccountPassword", failure);
     }
 
     private static GatewayOptions WithSecurity(SecurityOptions security)