test(secrets): serialize env-var-mutating test classes into one xunit collection

Environment.SetEnvironmentVariable/GetEnvironmentVariable mutate the
process-wide environ; concurrent setenv/getenv across parallel xunit test
classes is a documented crash/flake source on glibc even with unique
variable names. Group the six classes that touch process env vars into a
DisableParallelization=true collection so they run serially against each
other while everything else keeps running in parallel.

ZB.MOM.WW.Secrets/tests/ZB.MOM.WW.Secrets.Tests/Cli/Interactive/InteractiveShellTests.cs

@@ -13,6 +13,7 @@ namespace ZB.MOM.WW.Secrets.Tests.Cli.Interactive;
 /// KEK-upgrade prompt, and the error-containment panel. The shell is the only UI-owning class, so these
 /// tests assert on rendered <see cref="TestConsole.Output"/> and on what a fake flow observed.
 /// </summary>
+[Collection("Process environment")]
 public sealed class InteractiveShellTests : IDisposable
 {
     private readonly string _dir =

ZB.MOM.WW.Secrets/tests/ZB.MOM.WW.Secrets.Tests/Cli/Interactive/LiteralMasterKeyProviderTests.cs

@@ -11,6 +11,7 @@ namespace ZB.MOM.WW.Secrets.Tests.Cli.Interactive;
 /// <see cref="IMasterKeyProvider.KekId"/> derived identically to the configured providers so a key
 /// entered by hand can decrypt (and re-wrap) rows another provider kind sealed.
 /// </summary>
+[Collection("Process environment")]
 public sealed class LiteralMasterKeyProviderTests
 {
     private static string NewKeyBase64()

ZB.MOM.WW.Secrets/tests/ZB.MOM.WW.Secrets.Tests/Cli/Interactive/ReferenceAuditorTests.cs

@@ -13,6 +13,7 @@ namespace ZB.MOM.WW.Secrets.Tests.Cli.Interactive;
 /// (present/decryptable → Ok, absent → Missing, tombstoned → Tombstoned, wrong-KEK → Undecryptable),
 /// and the degraded-session presence-only downgrade (present → PresentUnverified).
 /// </summary>
+[Collection("Process environment")]
 public sealed class ReferenceAuditorTests : IDisposable
 {
     private readonly string _dir =

ZB.MOM.WW.Secrets/tests/ZB.MOM.WW.Secrets.Tests/Cli/Interactive/SecretsSessionFactoryTests.cs

@@ -13,6 +13,7 @@ namespace ZB.MOM.WW.Secrets.Tests.Cli.Interactive;
 /// (KEK unresolvable → degraded session that still lists metadata), the operator-override upgrade, and
 /// the SQL-Server-with-unresolved-secret-ref fall-back to the local store.
 /// </summary>
+[Collection("Process environment")]
 public sealed class SecretsSessionFactoryTests : IDisposable
 {
     private readonly string _dir =