Fix Voxtral Metal streaming mask

[JacobSzwejbka]

This changes Voxtral Realtime streaming Metal additive masks to use a finite large negative value instead of -inf, matching the existing offline Metal mask behavior. The failing Metal AOTI Voxtral CI run completed inference but generated no printable transcript after the PyTorch 2.12 bump; using a finite mask avoids MPS SDPA/AOTI propagating bad values from -inf masked positions.

Test plan:
PYTHONPATH=src python -m pytest examples/models/voxtral_realtime/tests/test_ring_kv_cache.py -q

This PR was authored with Claude.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19695

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❌ 1 Cancelled Job, 2 Unrelated Failures, 1 Unclassified Failure

As of commit dd8bf36 with merge base 7724fd7 ():

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• pull / unittest-editable / macos / macos-job (gh)

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• Test Metal Backend / test-model-metal-e2e (mistralai, Voxtral-Mini-4B-Realtime-2602, quantized-int4-metal) / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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