Run wgsl_template Python tests in CI (#32214)
### Description
<!-- Describe your changes. -->
Add a GitHub Actions workflow that runs the wgsl_template test suite on
Linux and Windows. The tests are stdlib-only, so no dependency install
is needed. Python 3.12 matches the WebGPU build lanes that invoke
wgsl_gen.py.
Remove `add_test()` block in cmake/onnxruntime_providers_webgpu.cmake.
The workflow calls `run_tests.py` directly.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Run wgsl_template tests in CI build.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
