taskPayloadSharedEXT qualifier misplaced leads to silent issue

The following (https://godbolt.org/z/oEGWT9Mh1)

```glsl
#version 460
#extension GL_EXT_mesh_shader : require

struct MyPayload {
float x;
};

taskPayloadSharedEXT MyPayload myPayloadData;

void main() {
myPayloadData.x = 0.0;
EmitMeshTasksEXT(1, 1, 1);
}
```

will produce `OpEmitMeshTasksEXT %uint_1 %uint_1 %uint_1 %myPayloadData` but if you go (https://godbolt.org/z/xTjWGv8rW)

```glsl
taskPayloadSharedEXT struct MyPayload {
float x;
};

MyPayload myPayloadData;
```

it will be missing the last operand in `OpEmitMeshTasksEXT`

... basically it should be invalid to use `taskPayloadSharedEXT` on a struct definition and an error should have been raised

Reproduce both GLSL snippets using the linked Compiler Explorer examples and compare the generated OpEmitMeshTasksEXT operands. Trace mesh-shader qualifier parsing, validation, and SPIR-V emission in glslang, then add coverage showing that the struct-definition form is rejected rather than emitting an incomplete instruction.

Written by the indexing model from the issue text.