Fixup navigations to/from join entity when new many-to-many relationship is created

Fixes #26779

Also cleanup of delay-fixup/suspended detection logic, and tests for #26074.

Author: ajcvickers

src/EFCore/ChangeTracking/Internal/ChangeDetector.cs

@@ -17,7 +17,6 @@ public class ChangeDetector : IChangeDetector
 {
     private readonly IDiagnosticsLogger<DbLoggerCategory.ChangeTracking> _logger;
     private readonly ILoggingOptions _loggingOptions;
-    private bool _suspended;
 
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
@@ -33,24 +32,6 @@ public ChangeDetector(
         _loggingOptions = loggingOptions;
     }
 
-    /// <summary>
-    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
-    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
-    ///     any release. You should only use it directly in your code with extreme caution and knowing that
-    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
-    /// </summary>
-    public virtual void Suspend()
-        => _suspended = true;
-
-    /// <summary>
-    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
-    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
-    ///     any release. You should only use it directly in your code with extreme caution and knowing that
-    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
-    /// </summary>
-    public virtual void Resume()
-        => _suspended = false;
-
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
     ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
@@ -59,8 +40,7 @@ public virtual void Resume()
     /// </summary>
     public virtual void PropertyChanged(InternalEntityEntry entry, IPropertyBase propertyBase, bool setModified)
     {
-        if (_suspended
-            || entry.EntityState == EntityState.Detached
+        if (entry.EntityState == EntityState.Detached
             || propertyBase is IServiceProperty)
         {
             return;
@@ -103,8 +83,7 @@ private static void ThrowIfKeyChanged(InternalEntityEntry entry, IProperty prope
     /// </summary>
     public virtual void PropertyChanging(InternalEntityEntry entry, IPropertyBase propertyBase)
     {
-        if (_suspended
-            || entry.EntityState == EntityState.Detached
+        if (entry.EntityState == EntityState.Detached
             || propertyBase is IServiceProperty)
         {
             return;

src/EFCore/ChangeTracking/Internal/IChangeDetector.cs

@@ -48,20 +48,4 @@ public interface IChangeDetector
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
     void DetectChanges(InternalEntityEntry entry);
-
-    /// <summary>
-    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
-    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
-    ///     any release. You should only use it directly in your code with extreme caution and knowing that
-    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
-    /// </summary>
-    void Suspend();
-
-    /// <summary>
-    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
-    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
-    ///     any release. You should only use it directly in your code with extreme caution and knowing that
-    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
-    /// </summary>
-    void Resume();
 }

src/EFCore/ChangeTracking/Internal/INavigationFixer.cs

@@ -23,23 +23,23 @@ public interface INavigationFixer
     ///     any release. You should only use it directly in your code with extreme caution and knowing that
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
-    void BeginAttachGraph();
+    bool BeginDelayedFixup();
 
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
     ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
     ///     any release. You should only use it directly in your code with extreme caution and knowing that
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
-    void CompleteAttachGraph();
+    void CompleteDelayedFixup();
 
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
     ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
     ///     any release. You should only use it directly in your code with extreme caution and knowing that
     ///     doing so can result in application failures when updating to a new Entity Framework Core release.
     /// </summary>
-    void AbortAttachGraph();
+    void AbortDelayedFixup();
 
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to