Metal CI: Add Qwen 3.5 MoE tiny model integration test

[manuelcandales]

Export the tiny model with --backend metal, build the C++ runner, and verify both decode (T=1) and prefill (T>2) complete successfully. Uses a byte-level tokenizer matching the tiny model's vocab_size=256.

Authored with Claude.

[pytorch-bot]

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❌ 3 New Failures, 1 Cancelled Job, 6 Pending, 4 Unrelated Failures

As of commit 2330a5a with merge base 6be4fb5 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv2d.py::TestConv2d::test_fp16_conv2d
• trunk / test-models-macos-coreml (resnet50) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos-coreml (vit) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• Test Metal Backend / test-model-metal-e2e (nvidia, parakeet-tdt, quantized-int4-metal) / macos-job (gh)

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

• trunk / test-qnn-model (fp32, mb) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[Copilot]

Pull request overview

Adds a new Metal CI integration test job that exports and runs the Qwen 3.5 MoE tiny model on macOS Metal, aiming to validate both prefill and decode execution paths.

Changes:

• Extend Metal workflow PR path filters to include the Qwen3.5 MoE example and LLM export code.
• Add a new test-metal-qwen35-moe-tiny job that installs ExecuTorch, exports a tiny Qwen3.5 MoE model for Metal, builds the runner, generates a minimal tokenizer, and runs two inference scenarios.

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