fix(test): opt all EventManager tests into NonParallel collection to prevent thread-safety race

EventManager.Reset() creates a fresh Lazy<EventManager> backing the singleton,
but the switch is not atomic across threads. Under xUnit's default parallel
class execution, one test can call Reset() between another test's
EventManager.Instance invocations, causing listeners to be registered on
one instance and dispatched on another — producing empty callback assertions.

Fixes flaky failures observed on Ubuntu CI runners in CoreTest where the
test output is nondeterministic due to OS-level scheduling differences.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: JusterZhu

tests/CoreTest/Bootstrap/ParameterMatrixAndEventTests.cs

@@ -22,6 +22,7 @@ namespace CoreTest.Bootstrap
     ///   - Various encoding/format combinations
     ///   - UpdateRequest validation edge cases
     /// </summary>
+    [Collection("NonParallel_EventManager")]
     public class ParameterMatrixAndEventTests : IDisposable
     {
         private readonly string _testDir;

tests/CoreTest/Event/EventListenerBatchTests.cs

@@ -9,6 +9,7 @@ namespace CoreTest.Event;
 /// Batch tests for <see cref="IUpdateEventListener"/> and <see cref="EventManager"/>
 /// following AAAT (Arrange-Act-Assert-TearDown).
 /// </summary>
+[Collection("NonParallel_EventManager")]
 public class EventListenerBatchTests : IDisposable
 {
     /// <summary>TearDown: clear singleton state after each test for isolation.</summary>

tests/CoreTest/Event/EventListenerTests.cs

@@ -6,6 +6,7 @@
 
 namespace CoreTest.Event;
 
+[Collection("NonParallel_EventManager")]
 public class EventListenerTests : IDisposable
 {
     /// <summary>TearDown: clear singleton state after each test for isolation.</summary>

tests/CoreTest/Event/EventManagerConcurrencyTests.cs

@@ -6,6 +6,7 @@
 
 namespace CoreTest.Event;
 
+[Collection("NonParallel_EventManager")]
 public class EventManagerConcurrencyTests : IDisposable
 {
     /// <summary>TearDown: clear singleton state after each test for isolation.</summary>

tests/CoreTest/Event/EventManagerTests.cs

@@ -7,6 +7,7 @@ namespace CoreTest.Event;
 /// Implements <see cref="IDisposable"/> for explicit TearDown — clears singleton state
 /// after each test to ensure test isolation regardless of test execution order.
 /// </summary>
+[Collection("NonParallel_EventManager")]
 public class EventManagerTests : IDisposable
 {
     public class TestEventArgs : EventArgs