[webgpu] Enable im2col-matmul for newer Xe platforms; limit to fp16 (#29505)
### Description
- Add xe-2hpg, xe-3lpg, and xe-3lpg-xs to the execution allowlist.
- Gate im2col-matmul on fp16 as the kernel is tuned for it.
- Fall back to default convolution for fp32 and other data types.
### Motivation and Context
See above.