[WebGPU] Add MRotaryEmbedding support (#31976)
## Description
Add WebGPU Execution Provider support for the Microsoft-domain
MRotaryEmbedding contrib op introduced for CPU and CUDA in #31728.
The WebGPU kernel:
- supports rank-3 BSNH and rank-4 BNSH inputs
- supports sectioned and interleaved mRoPE layouts
- supports interleaved and non-interleaved rotary pairing
- supports float and float16 data, scaling, and partial rotary
dimensions
- preserves non-rotary tail values and invalid/out-of-range position IDs
- validates full int64 position IDs by reading both u32 storage words
- implements the shader as a parameterized WGSL template
The operator tests now explicitly exercise WebGPU and extend the
formerly CUDA-only negative-attribute, out-of-bounds position ID, and
empty-input cases to both GPU providers.
## Testing
- Release WebGPU build of onnxruntime_provider_test
- onnxruntime_provider_test.exe
--gtest_filter="ContribOpMRotaryEmbeddingTest.*" (9 tests passed)
- WGSL template generation across the WebGPU source tree (52 templates
generated)
- lintrunner on all changed C++, header, test, and WGSL template files
- git diff --check
## Notes
The standalone WGSL template Python suite has two pre-existing
golden-file mismatches for core subgroup-matrix templates. That smoke
test scans core/providers/webgpu only; this PR adds its template under
contrib_ops/webgpu and does not affect those mismatches.
