[webgpu] Optimize LinearAttention Op with subgroup (#28412)
### Description
Optimize the `LinearAttention` Op with subgroupAdd().
- Detect the adapter's `subgroupMinSize` to allocate workgroup shared
memory
- Drastically reduces workgroup shared memory usage (workgroup_size_x *
TILE_V → MAX_SG * TILE_V)
- Eliminates most `workgroupBarrier()` calls in the subgroup reduction
The optimization is gated behind `subgroup_min_size`, which is enabled
when the device supports `wgpu::FeatureName::Subgroups`.
The original is preserved as fallback.
## Qwen3.5-4B Performance Benchmarks
| Metric | Prefill Speed (TPS) | Decode Speed (TPS) | Prefill
Improvement |
| :--- | :--- | :--- | :--- |
| **Default** | 719.6 | 29.6 | - |
| **Optimized** | 929.8 | 29.7 | 1.29x |
**Test Environment:**
* **Hardware:** Intel Panther Lake
* **Configuration:** Prefill: 1024, Decode: 128
### Motivation and Context
See above.
