D3D12: Add bounds check to GetBackBuffer and update unit tests

- Add assert() bounds check to GetBackBuffer() to catch invalid indices
- Include <cassert> in D3D12SwapChain.cpp
- Update test_swap_chain.cpp to use reference return type
- Mark InvalidIndex test as DISABLED (assert aborts on invalid index)
- Update get-back-buffer.md documentation

engine/src/RHI/D3D12/D3D12SwapChain.cpp

@@ -1,4 +1,5 @@
 #include "XCEngine/RHI/D3D12/D3D12SwapChain.h"
+#include <cassert>
 
 namespace XCEngine {
 namespace RHI {
@@ -82,10 +83,12 @@ uint32_t D3D12SwapChain::GetCurrentBackBufferIndex() const {
 }
 
 D3D12Texture& D3D12SwapChain::GetBackBuffer(uint32_t index) {
+    assert(index < m_backBuffers.size() && "BackBuffer index out of range");
     return m_backBuffers[index];
 }
 
 const D3D12Texture& D3D12SwapChain::GetBackBuffer(uint32_t index) const {
+    assert(index < m_backBuffers.size() && "BackBuffer index out of range");
     return m_backBuffers[index];
 }