test(host): supply Central test ApiKeyPepper so StartupValidator preflight passes (fix pre-existing 1fcc4f5 red); lock pepper-required behavior

Commit 1fcc4f5 added a Central-only Require for ScadaBridge:InboundApi:ApiKeyPepper
(>=16 chars) to StartupValidator. That Require fires in Program.cs before WebApplicationFactory
can apply any WithWebHostBuilder config overlays, so it must be satisfied via environment
variables (which ARE in the pre-host AddEnvironmentVariables() pass).

Fix (test-only, no src/ changes):
- CentralDbTestEnvironment: add ScadaBridge__InboundApi__ApiKeyPepper env var (TestPepper
  constant, 23 chars) alongside the existing db connection string; restore on Dispose.
  Fixes HealthCheckTests, MetricsEndpointTests, and HostStartupTests.CentralRole_StartsWithoutError
  which all use CentralDbTestEnvironment.
- CentralActorPathTests.InitializeAsync: set the pepper env var before WebApplicationFactory
  is constructed (the class uses IAsyncLifetime directly, not CentralDbTestEnvironment).
- CentralCompositionRootTests ctor + Dispose: same env-var pattern; those tests already had
  the pepper in AddInMemoryCollection (DI-layer only, too late for pre-host validation).
- CentralAuditWiringTests ctor + Dispose: same env-var pattern for the same reason.
- StartupValidatorTests.ValidCentralConfig(): add pepper so the unit tests that call
  StartupValidator.Validate() directly with a Central config stop failing.
- Add guard tests: Central_MissingApiKeyPepper_FailsValidation,
  Central_ShortApiKeyPepper_FailsValidation, Site_ApiKeyPepper_NotRequired — these lock
  the production behavior introduced by 1fcc4f5.

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

@@ -8,7 +8,10 @@ namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
 /// environment variable (<c>Program</c>'s configuration builder calls
 /// <c>AddEnvironmentVariables()</c>).
 ///
-/// Dispose restores the previous value so tests stay isolated.
+/// Also supplies <c>ScadaBridge__InboundApi__ApiKeyPepper</c> so the Central-role
+/// StartupValidator preflight (added in 1fcc4f5) does not fail for tests that set
+/// <c>DOTNET_ENVIRONMENT=Central</c> without an explicit pepper env var. Both vars
+/// are restored on Dispose so tests stay isolated.
 /// </summary>
 internal sealed class CentralDbTestEnvironment : IDisposable
 {
@@ -19,16 +22,27 @@ internal sealed class CentralDbTestEnvironment : IDisposable
 
     private const string ConfigKey = "ScadaBridge__Database__ConfigurationDb";
 
+    // Test-only pepper — satisfies the ≥16-char StartupValidator requirement without
+    // committing a real secret. The env-var name uses the double-underscore delimiter
+    // so AddEnvironmentVariables() maps it to ScadaBridge:InboundApi:ApiKeyPepper.
+    internal const string TestPepper = "test-pepper-01234567890";
+    private const string PepperKey = "ScadaBridge__InboundApi__ApiKeyPepper";
+
     private readonly string? _previousConfig;
+    private readonly string? _previousPepper;
 
     public CentralDbTestEnvironment()
     {
         _previousConfig = Environment.GetEnvironmentVariable(ConfigKey);
         Environment.SetEnvironmentVariable(ConfigKey, ConfigurationDb);
+
+        _previousPepper = Environment.GetEnvironmentVariable(PepperKey);
+        Environment.SetEnvironmentVariable(PepperKey, TestPepper);
     }
 
     public void Dispose()
     {
         Environment.SetEnvironmentVariable(ConfigKey, _previousConfig);
+        Environment.SetEnvironmentVariable(PepperKey, _previousPepper);
     }
 }