RHI: Add embedded shader source support via ShaderCompileDesc

- Add ShaderLanguage enum (HLSL, GLSL, SPIRV)
- Extend ShaderCompileDesc with source/sourceLanguage fields
- D3D12Device::CompileShader supports both file and embedded source
- OpenGLDevice::CompileShader supports embedded GLSL source
- Refactor test_shader.cpp to use embedded source for both backends

This enables consistent shader compilation across D3D12 and OpenGL
backends while maintaining backend-specific shader language support.

engine/include/XCEngine/RHI/RHIEnums.h

@@ -19,6 +19,13 @@ enum class ShaderType : uint8_t {
     Library
 };
 
+enum class ShaderLanguage : uint8_t {
+    Unknown,
+    HLSL,
+    GLSL,
+    SPIRV
+};
+
 enum class CullMode : uint8_t {
     None,
     Front,

engine/include/XCEngine/RHI/RHITypes.h

@@ -43,10 +43,14 @@ struct ShaderCompileMacro {
     std::wstring definition;
 };
 
+enum class ShaderLanguage : uint8_t;
+
 struct ShaderCompileDesc {
+    std::wstring fileName;
+    std::vector<uint8_t> source;
+    ShaderLanguage sourceLanguage = ShaderLanguage::Unknown;
     std::wstring entryPoint;
     std::wstring profile;
-    std::wstring fileName;
     std::vector<ShaderCompileMacro> macros;
 };