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/CompositionRootTests.cs

@@ -95,6 +95,11 @@ public class CentralCompositionRootTests : IDisposable
         // runs before WithWebHostBuilder.ConfigureAppConfiguration applies DI config.
         Environment.SetEnvironmentVariable("ScadaBridge__InboundApi__ApiKeyPepper",
             CentralDbTestEnvironment.TestPepper);
+        // Supply MachineDataDb so the reverted Host-008 Require (REQ-HOST-3/4, M2.9 #17)
+        // passes for Central-role StartupValidator. A non-empty placeholder satisfies
+        // the preflight; the DI override below replaces the real DbContext anyway.
+        Environment.SetEnvironmentVariable("ScadaBridge__Database__MachineDataDb",
+            "Server=localhost;Database=MachineData;");
 
         _factory = new WebApplicationFactory<Program>()
             .WithWebHostBuilder(builder =>
@@ -159,6 +164,7 @@ public class CentralCompositionRootTests : IDisposable
         _factory.Dispose();
         Environment.SetEnvironmentVariable("DOTNET_ENVIRONMENT", _previousEnv);
         Environment.SetEnvironmentVariable("ScadaBridge__InboundApi__ApiKeyPepper", null);
+        Environment.SetEnvironmentVariable("ScadaBridge__Database__MachineDataDb", null);
     }
 
     // --- Singletons ---