ShaderTool reports only error count, not per-line diagnostics, on shader compile failure

[Filed by Copilot on behalf of @bghgary]

## Summary

When a shader fails to compile, `ShaderTool` (and the underlying `ShaderCompiler`) reports only glslang's summary line and discards the per-line diagnostics. This makes shader compile failures very hard to debug — there is no line number, no offending identifier, and no indication of which shader (vertex vs fragment) failed.

## Repro

Compile a fragment shader containing a compile error (here, an undeclared identifier `uLayer`) with `ShaderTool`:

```glsl
#define SHADER_NAME fragment:composition
precision highp float;
uniform sampler2D plane0Sampler;
in vec2 vUV;
layout(location = 0) out vec4 glFragColor;
void main() { glFragColor = texture(plane0Sampler, vec3(vUV, uLayer)); }
```

```
ShaderTool -o out.bin vertex.fx fragment.fx
```

## Observed

```
Compiling: vertex.fx + fragment.fx
Error: ERROR: 3 compilation errors. No code generated.
```

That summary line is the *entire* error output. glslang clearly detected 3 specific errors but none of the per-line detail is surfaced.

## Expected

The per-line glslang diagnostics, e.g.:

```
ERROR: 0:6: 'uLayer' : undeclared identifier
ERROR: 0:6: 'texture' : no matching overloaded function found
ERROR: 0:6: '=' : cannot convert ...
```

…plus which shader stage / file the failure came from.

## Where

`Plugins/ShaderCompiler/Source/ShaderCompiler{DXBC,DXIL,Metal,OpenGL,Vulkan}.cpp`, `AddShader()`:

```cpp
if (!shader.parse(defaultTBuiltInResource, 310, EProfile::EEsProfile, true, true, EShMsgDefault))
{
throw std::runtime_error{shader.getInfoLog()};
}
```

In this case `shader.getInfoLog()` returns only the summary line. Worth investigating whether the per-line detail is available via `getInfoDebugLog()` and/or a different `EShMessages` flag, and including the failing shader stage/path in the thrown message (the `Compiling: …` line in `ShaderTool` is the only hint today, and even that doesn't disambiguate vertex vs fragment).