[ExecuTorch][WebGPU] Consolidate landed-op tests into the cases.py op-test framework

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Add mul op with full broadcast (aten.mul.Tensor) #20358
• -> [ExecuTorch][WebGPU] Consolidate landed-op tests into the cases.py op-test framework #20357
• [ExecuTorch][WebGPU] Op-test codegen framework (cases.py -> generated .pte+golden -> gtest driver) #20339

add and rms_norm already have declarative suites in the cases.py framework, so their standalone tests are redundant. Remove them, leaving the framework as the single home for these stateless single-forward ops (mirroring Vulkan). update_cache and sdpa stay hand-written — stateful KV-cache replay can't be a single module -> golden forward.

Removed:

• test_single_add/test_chained_add + their main() plumbing from test_webgpu_native.cpp.
• the standalone test/native/test_rms_norm.cpp binary + its CMake target.
• the add/rms_norm export+run wiring in test_webgpu_native_ci.sh + test_build_webgpu.sh.
• webgpu_native_test re-gated on the executorch wheel being importable (was the add .pte); it still hosts the quantized_linear/SDPA/update_cache/symint sweeps.
  @exported-using-ghexport

Differential Revision: D108821384

Differential Revision: D108821384

[pytorch-bot]

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❌ 4 New Failures, 6 Pending, 1 Unrelated Failure

As of commit f3bc481 with merge base eb7473b ():

NEW FAILURES - The following jobs have failed:

• Propose to merge ghstack orig PRs to main / Try to create a PR with ghstack /orig branch (gh)
  Process completed with exit code 1.
• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t deced19036a44651075ac957b1d1afdefc42f0a6532889ad9ed2a76b502f4b64 /exec failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 2020ffdad73353c381fa9f44184691a7cee389fb03d355702c45c0f155cd8d33 /exec failed with exit code 1
• pull / test-voxtral-realtime-xnnpack-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 94df99483bdd6306f362109c7de50411ced47defd4b34883801d8326d9028949 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-moshi-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/m/mesa/mesa-vdpau-drivers_23.2.1-1ubuntu3.1%7e22.04.3_amd64.deb 404 Not Found [IP: 104.20.28.246 80]

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This pull request has been merged in 86887ec.