Refactor shader management and remove unused shaders

- Introduced a shader toolkit for dynamic shader generation in soundboard.lua.
- Removed obsolete shader files: ceiling.frag, cube.frag, cube.vert, floor.frag, pbr.frag, pbr.vert, solid.frag, solid.vert, wall.frag.
- Updated pipeline_service to handle inline shader sources and paths, improving shader registration and validation.
- Enhanced shader_script_service to support optional shader source paths alongside traditional paths.
- Modified ShaderPaths structure to include source fields for vertex, fragment, geometry, tessellation control, tessellation evaluation, and compute shaders.
- Updated test_gxm_backend to reflect changes in shader paths and ensure successful pipeline creation.

docs/test_error_handling.md

@@ -83,7 +83,7 @@ The file exists but cannot be opened. Check file permissions.
 
 **Example error message**:
 ```
-Vertex shader not found: shaders/cube.vert
+Vertex shader not found: shaders/gui_2d.vert
 
 Shader key: default
 
@@ -160,14 +160,14 @@ Plus a message box with the same error.
 ### Test Case 2: Missing Shader File
 ```bash
 cd build/Release
-mv shaders/cube.vert shaders/cube.vert.backup
-./sdl3_app --json-file-in ./config/seed_runtime.json
+mv shaders/gui_2d.vert shaders/gui_2d.vert.backup
+./sdl3_app --json-file-in ./config/gui_runtime.json
 ```
 
 **Expected Result**:
 Message box showing:
 ```
-Vertex shader not found: shaders/cube.vert
+Vertex shader not found: shaders/gui_2d.vert
 
 Shader key: default