test: add runtime atomics and malformed frontmatter coverage

[dowdiness]

Summary

• Add execution-level regression coverage for Atomics timeout coercion/capping in test262 harness preamble.
  • New async whitebox test "atomics short timeout wrapper passes coerced timeouts at runtime" in tooling/test262_runner/runner_wbtest.mbt.
  • Runs the generated short-profile harness in Node and verifies Atomics.wait timeout arguments are normalized as expected.
• Extend fallback regression coverage for malformed frontmatter metadata.
  • Existing fallback test now includes a malformed frontmatter fixture and asserts test task fallback still expands to both strict/non-strict modes.

Validation

• moon test --target native tooling/test262_runner (9/9)
• moon test --target native (2183/2183)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ad7f4803-ae17-4a69-aa38-bb2db18538e5

📥 Commits

Reviewing files that changed from the base of the PR and between 3f49b1e and d8d5f5f.

📒 Files selected for processing (1)

• tooling/test262_runner/runner_wbtest.mbt

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/test262-atomics-runtime-followup

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[chatgpt-codex-connector]

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