Enforce mesh shader output requirements

Description
Currently, mesh shader is not requiring output vertices or indices as should be required.

There should also be a requirement that mesh shader vertex output has SV_Position (not enforced currently either). With that were to be enforced, and added to the struct, the validator will fail unless you write to all components of SV_Position. This should probably only be the case if OutputCounts are set to anything other than 0.

While shaders without these components aren't very useful in practice, a lot of our testing has cut corners like this and will have to be updated as well.

Steps to Reproduce
Just create a mesh shader with no output vertices nor indices.

https://godbolt.org/z/4fc65rGqq

Actual Behavior
The shader compiles, but it shouldn't as such a shader is incorrect and not at all useful.

The validator needs to catch this case.

Environment

• DXC version 1.7.0.4313
• Host Operating System win11

Start with the mesh shader validator and the Godbolt reproduction linked in the issue. Trace how output vertices, indices, OutputCounts, and SV_Position are currently checked, then update the affected shader tests, including cases that omit these outputs. Done means invalid mesh shaders are rejected while valid output-count-zero cases continue to compile.