test(host): serialize Central-boot fixtures to close env-var race

CentralDbTestEnvironment sets five process-wide environment variables, and
Program's AddEnvironmentVariables() reads them at an unpredictable point during
host boot. With xUnit collection parallelization on, one fixture's teardown
could clear a var mid-boot for a sibling. Since the secrets adoption (G-4)
three of those keys are ${secret:...} references that fail closed, turning a
previously benign empty value into a SecretNotFoundException that aborts the
boot — an intermittent CI failure.

Adds a HostBootCollection that serializes every fixture booting a real host
while depending on that shared state, folding in the narrower "ActorSystem"
collection so its members stay serialized with each other as before. Site-role
fixtures stay parallel: they call Configuration.Sources.Clear(), dropping the
env-var provider, so they cannot participate in the race.

CentralDbTestEnvironment now also fails fast if two instances are ever live at
once, making a regression (a fixture added outside the collection) deterministic
rather than intermittent — this is what surfaced the disposed-CTS defect fixed
in the previous commit. Fixture teardown is try/finally so a throwing host
teardown can no longer strand the vars for the rest of the run.

Cost: Host.Tests runs ~2m30s -> ~4m10s; the serialized Central-boot classes are
most of the assembly's parallelism.

Fixes: Gitea #15

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

@@ -10,14 +10,11 @@ using ZB.MOM.WW.ScadaBridge.Host.Actors;
 
 namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
 
-[CollectionDefinition("ActorSystem")]
-public class ActorSystemCollection : ICollectionFixture<object> { }
-
 /// <summary>
 /// Verifies that all expected Central-role actors are created at the correct paths
 /// when AkkaHostedService starts.
 /// </summary>
-[Collection("ActorSystem")]
+[Collection(HostBootCollection.Name)]
 public class CentralActorPathTests : IAsyncLifetime
 {
     private WebApplicationFactory<Program>? _factory;
@@ -93,9 +90,20 @@ public class CentralActorPathTests : IAsyncLifetime
 
     public async Task DisposeAsync()
     {
-        _factory?.Dispose();
-        Environment.SetEnvironmentVariable("DOTNET_ENVIRONMENT", _previousEnv);
-        _dbEnv?.Dispose();
+        // try/finally: a throwing host teardown must not strand the process-wide
+        // environment variables. Without it, one failed _factory.Dispose() leaks
+        // DOTNET_ENVIRONMENT=Central and the CentralDbTestEnvironment vars into
+        // every later test in the run.
+        try
+        {
+            _factory?.Dispose();
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable("DOTNET_ENVIRONMENT", _previousEnv);
+            _dbEnv?.Dispose();
+        }
+
         await Task.CompletedTask;
     }
 
@@ -149,7 +157,7 @@ public class CentralActorPathTests : IAsyncLifetime
 /// Verifies that all expected Site-role actors are created at the correct paths
 /// when AkkaHostedService starts.
 /// </summary>
-[Collection("ActorSystem")]
+[Collection(HostBootCollection.Name)]
 public class SiteActorPathTests : IAsyncLifetime
 {
     private IHost? _host;