Handle allocator propagation in basic_memory_buffer::move, Fix #4487 (#4490)

* Handle allocator propagation in basic_memory_buffer::move

Update `basic_memory_buffer::move` to respect `propagate_on_container_move_assignment`allocator trait.
If the allocator should not propagate and differs from the target's allocator,
fallback to copying the buffer instead of transferring ownership.

This avoids potential allocator mismatch issues and ensures exception safety.

* Add test cases for the updated move ctor

- Added two test cases `move_ctor_inline_buffer_non_propagating` and `move_ctor_dynamic_buffer_non_propagating`
- Added `PropageteOnMove` template parameter to `allocator_ref` class to be compatible with the old test cases
- `allocator_ref` now implements `!=` and `==` operators

test/mock-allocator.h

@@ -37,7 +37,8 @@ template <typename T> class mock_allocator {
   MOCK_METHOD(void, deallocate, (T*, size_t));
 };
 
-template <typename Allocator> class allocator_ref {
+template <typename Allocator, bool PropagateOnMove = false>
+class allocator_ref {
  private:
   Allocator* alloc_;
 
@@ -48,6 +49,9 @@ template <typename Allocator> class allocator_ref {
 
  public:
   using value_type = typename Allocator::value_type;
+  using propagate_on_container_move_assignment =
+      typename std::conditional<PropagateOnMove, std::true_type,
+                                std::false_type>::type;
 
   explicit allocator_ref(Allocator* alloc = nullptr) : alloc_(alloc) {}
 
@@ -72,6 +76,19 @@ template <typename Allocator> class allocator_ref {
     return std::allocator_traits<Allocator>::allocate(*alloc_, n);
   }
   void deallocate(value_type* p, size_t n) { alloc_->deallocate(p, n); }
+
+  FMT_CONSTEXPR20 friend bool operator==(const allocator_ref& a,
+                                         const allocator_ref& b) noexcept {
+    if (a.alloc_ == b.alloc_) return true;
+    if (a.alloc_ == nullptr || b.alloc_ == nullptr) return false;
+
+    return *a.alloc_ == *b.alloc_;
+  }
+
+  FMT_CONSTEXPR20 friend bool operator!=(const allocator_ref& a,
+                                         const allocator_ref& b) noexcept {
+    return !(a == b);
+  }
 };
 
 #endif  // FMT_MOCK_ALLOCATOR_H_