test+fix: eliminate Windows full-suite temp-file-lock flakiness

Full-host tests passed in isolation but failed 2-4/run under parallel execution on
Windows with 'the process cannot access the file ... because it is being used by
another process' (macOS never sees it — Unix deletes open files). Two shared-state
parallel collisions, discovered while verifying TST-08:

1. Self-signed cert generation. GatewayTlsBootstrapTests sets process-global
   Kestrel/TLS env vars that GatewayApplication.Build reads; a parallel host-building
   test inherits them mid-run and both generate a cert at the same path, racing on the
   fixed-name '<path>.tmp'. Fixes:
   - SelfSignedCertificateProvider: stage the PFX in a unique '<path>.<guid>.tmp' instead
     of a fixed name, so concurrent/interrupted writers never collide on the temp file
     (real robustness: two instances or a restart-during-write no longer clash). The
     final atomic Move (last-writer-wins) still yields an equivalent cert.
   - TestHostEnvironmentInitializer: default MxGateway__Tls__SelfSignedCertPath to a
     per-process temp path so the suite never writes the shared ProgramData default or
     fights the deployed service; first host-building test generates, the rest load it.
   - GatewayTlsBootstrapTests: [Collection] with DisableParallelization so its global
     env-var mutation cannot bleed into parallel tests (new GlobalEnvironmentCollection).

2. AuthStoreHealthCheckTests opened a real SQLite file; Microsoft.Data.Sqlite's
   connection pool keeps the .db handle open after 'await using', so the finally
   File.Delete threw. Reuse the existing TempDatabaseDirectory helper, which calls
   SqliteConnection.ClearAllPools() before deleting.

Server build clean; affected classes 17/17 on macOS. Windows full-suite verified separately.

src/ZB.MOM.WW.MxGateway.Tests/TestSupport/TestHostEnvironmentInitializer.cs

@@ -3,7 +3,8 @@ using System.Runtime.CompilerServices;
 namespace ZB.MOM.WW.MxGateway.Tests.TestSupport;
 
 /// <summary>
-/// Defaults the host environment to Development for the whole test assembly.
+/// Defaults the host environment to Development and isolates the test process's on-disk
+/// gateway paths, for the whole test assembly.
 /// </summary>
 /// <remarks>
 /// Many tests build the full gateway host through <c>GatewayApplication.Build</c> against the dev
@@ -15,6 +16,15 @@ namespace ZB.MOM.WW.MxGateway.Tests.TestSupport;
 /// <c>GatewayOptionsValidator</c> with its <c>isProduction</c> constructor (no host environment) and
 /// are unaffected; a test that needs Production can still pass <c>--environment=Production</c>, which
 /// overrides this default.
+/// <para>
+/// The self-signed-cert path is also defaulted to a per-process temp file. Otherwise every
+/// full-host test that triggers HTTPS-cert generation writes the shared
+/// <c>CommonApplicationData/MxGateway/certs/gateway-selfsigned.pfx</c> default — parallel xUnit
+/// test classes then collide on that path's temp file (Windows: "the process cannot access the
+/// file ... because it is being used by another process"), and on a shared CI/dev box the suite
+/// also fights the deployed gateway service for the same file. Pointing at a per-process path
+/// isolates the suite: the first host-building test generates the cert, the rest load it.
+/// </para>
 /// </remarks>
 internal static class TestHostEnvironmentInitializer
 {
@@ -25,5 +35,17 @@ internal static class TestHostEnvironmentInitializer
         {
             Environment.SetEnvironmentVariable("ASPNETCORE_ENVIRONMENT", "Development");
         }
+
+        if (string.IsNullOrEmpty(Environment.GetEnvironmentVariable("MxGateway__Tls__SelfSignedCertPath")))
+        {
+            // ProcessId keeps the path stable within this test process (so a valid cert
+            // generated by the first host-building test is reused by the rest) yet unique
+            // across processes (so concurrent test runs / the deployed service never share it).
+            string certPath = Path.Combine(
+                Path.GetTempPath(),
+                $"mxgw-tests-{Environment.ProcessId}",
+                "gateway-selfsigned.pfx");
+            Environment.SetEnvironmentVariable("MxGateway__Tls__SelfSignedCertPath", certPath);
+        }
     }
 }