Fix ghost entry in EntityReferenceMap after identity conflict during Add

[Copilot]

ChangeTracker.HasChanges() could return false and SaveChanges() could return an incorrect row count after catching an identity conflict exception and detaching the offending entity.

Root cause

When Add(entity) throws an identity conflict, FireStateChanging had already moved the entry from the detached bucket to the added bucket in EntityReferenceMap. The outer SetEntityState correctly restores EntityState = Detached, but SetEntityState(Detached, Detached) is a no-op — so the entry is left as a ghost in _addedReferenceMap with EntityState.Detached. This ghost inflated SaveChanges row counts and corrupted ChangedCount.

context.Foos.Add(foo2);         // throws: identity conflict
context.Entry(foo2).State = EntityState.Detached;  // no-op — EntityState already Detached,
                                                    // but entry still in _addedReferenceMap
context.Bars.Add(new Bar());
context.ChangeTracker.HasChanges();  // false — ChangedCount was -1
context.SaveChanges();               // returned 2 instead of 1

Changes

• InternalEntityEntry.OnStateChanging — wrap StateManager.StartTracking(this) in try/catch; on failure, call StateManager.RevertStartTracking(this, newState) to undo the reference map move before rethrowing.
• IStateManager — new void RevertStartTracking(InternalEntityEntry entry, EntityState requestedState).
• StateManager — implements RevertStartTracking by delegating to the existing private UpdateReferenceMaps(entry, entry.EntityState, requestedState), which removes the entry from the wrongly-placed bucket without adding it to the detached map.
• FakeStateManager — stub to satisfy the interface.
• StateManagerTest — four regression tests covering HasChanges, ChangedCount, Entries enumeration, and SaveChanges row count after an identity conflict + detach.