[Bugfix][Quantization] Respect explicit WNA16 MoE backend

[aoshen02]

Summary

• honor an explicit moe_backend=triton for compressed-tensors WNA16 MoE
• keep auto and explicit marlin selecting Marlin when the layer is supported
• add focused coverage for all three selections

Root cause

The WNA16 selector chose Marlin whenever the layer supported it, before consulting kernel_config.moe_backend. As a result, an explicit Triton request was silently overridden.

Fix

Marlin is selected only when the requested backend is auto or marlin, the layer is supported, and the platform is not ROCm. Other paths continue through the existing WNA16 backend selection.

Validation

• full pre-commit passed
• focused test added for auto -> marlin, marlin -> marlin, and triton -> triton
• the focused pytest command could not run on the host because its system Python does not have pytest installed

AI assistance was used. The submitter reviewed every changed line.