fix(host): add MachineDataDb startup validation for Central (reverts Host-008, M2.9 #17)

REQ-HOST-3/REQ-HOST-4 require a MachineDataDb connection string for Central nodes.
The shipped docker appsettings (docker/central-node-a/appsettings.Central.json and
central-node-b) already carry the key. Host-008 had removed the fail-fast Require
because MachineDataDb had no consumer yet; this commit reverses that decision so a
misconfigured or missing connection string is caught at startup with a clear error.

Changes:
- DatabaseOptions: add MachineDataDb property with XML doc comment
- StartupValidator: add .Require for ScadaBridge:Database:MachineDataDb inside the
  existing Central .When block, immediately after the ConfigurationDb Require
- StartupValidatorTests: rename Central_MissingMachineDataDb_PassesValidation ->
  FailsValidation and flip to Assert.Throws; update comment to cite REQ-HOST-3/4,
  shipped docker appsettings, and the Host-008 reversal; add MachineDataDb to
  ValidCentralConfig() so all other Central tests remain green
- CentralDbTestEnvironment: supply ScadaBridge__Database__MachineDataDb env var
  (mirrors ConfigurationDb pattern) so HostStartupTests, HealthCheckTests, and
  MetricsEndpointTests pass through the new Require
- CompositionRootTests, AkkaHostedServiceAuditWiringTests, ActorPathTests: set
  ScadaBridge__Database__MachineDataDb env var alongside the pepper env var and
  clear it in Dispose, matching the existing pepper handling pattern

Build: 0 warnings, 0 errors. dotnet test Host.Tests: 233/233 passed.

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/StartupValidatorTests.cs

@@ -20,6 +20,7 @@ public class StartupValidatorTests
         ["ScadaBridge:Node:NodeHostname"] = "central-node1",
         ["ScadaBridge:Node:RemotingPort"] = "8081",
         ["ScadaBridge:Database:ConfigurationDb"] = "Server=localhost;Database=Config;",
+        ["ScadaBridge:Database:MachineDataDb"] = "Server=localhost;Database=MachineData;",
         ["ScadaBridge:Security:Ldap:Server"] = "ldap.example.com",
         ["ScadaBridge:Security:JwtSigningKey"] = "test-signing-key-at-least-32-chars-long",
         ["ScadaBridge:Cluster:SeedNodes:0"] = "akka.tcp://scadabridge@central-node1:8081",
@@ -152,17 +153,19 @@ public class StartupValidatorTests
     }
 
     [Fact]
-    public void Central_MissingMachineDataDb_PassesValidation()
+    public void Central_MissingMachineDataDb_FailsValidation()
     {
-        // Host-008 regression: MachineDataDb is never consumed anywhere in the
-        // system (only ConfigurationDb is wired into AddConfigurationDatabase).
-        // It is no longer a required key, so its absence must not fail startup.
+        // Reverts Host-008. REQ-HOST-3/REQ-HOST-4 require MachineDataDb to be
+        // validated at startup for Central nodes, and the shipped docker appsettings
+        // (docker/central-node-a/appsettings.Central.json and central-node-b) carry
+        // the key. The prior Host-008 decision (which removed the Require) is reversed
+        // here (#17, M2.9): a missing MachineDataDb must fail fast with a clear error.
         var values = ValidCentralConfig();
         values.Remove("ScadaBridge:Database:MachineDataDb");
         var config = BuildConfig(values);
 
-        var ex = Record.Exception(() => StartupValidator.Validate(config));
-        Assert.Null(ex);
+        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
+        Assert.Contains("MachineDataDb connection string required for Central", ex.Message);
     }
 
     [Fact]