Assert buffer sizes in test_workspace_sharing.cpp

[GregoryComer]

Summary

Add asserts to ensure that the backend option key field is large enough to contain the xnnpack workspace sharing key in test_workspace_sharing.cpp.

[pytorch-bot[bot]]

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:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15837

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:x: 1 Cancelled Job, 5 Unrelated Failures

As of commit c1ee50363160059e5770370aa13a5cb68db5166a with merge base a6c59218cf27ed25495bf34a0892710ebfe1e79b ():

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

• Build Presets / linux (pybind, linux.2xlarge, executorch-ubuntu-22.04-clang12) / build (gh) The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux (add_mul, portable, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-models-linux (mv2, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / unittest-buck / macos / macos-job (gh) (similar failure) RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

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

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

• pull / unittest / macos / macos-job (gh) (trunk failure) RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2
• pull / unittest-editable / macos / macos-job (gh) (trunk failure) RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

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