test: harden test262 tooling regression coverage

[dowdiness]

Summary

• Added regression coverage for Atomics timeout harness behavior under atomicsTimeoutProfile=short.
• Added a regression test for build_test_tasks default-mode fallback when metadata read/parsing fails.
• Migrated deprecated constructs in tooling tests:
  • try? → try/catch fallback in tooling/test262_runner/task_selection.mbt
  • String.substring(...) → view slicing in tooling/subprocess_helpers/report_test262.mbt

Validation

• moon check --target native tooling/test262_runner tooling/subprocess_helpers
• moon test --target native tooling/test262_runner tooling/subprocess_helpers
• moon test --target native (full repo): all tests pass

Summary by CodeRabbit

• Tests

  • Added test cases for error handling in test task building and atomics timeout normalization to improve test coverage.

• Refactor

  • Optimized internal shard-filename parsing logic to use efficient slice syntax.
  • Enhanced error handling in test task selection to gracefully fall back to defaults when metadata reading fails.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 7e189587-b09f-4791-a53a-a35e05240fb8

📥 Commits

Reviewing files that changed from the base of the PR and between ef35054 and f134106.

📒 Files selected for processing (3)

• tooling/subprocess_helpers/report_test262.mbt
• tooling/test262_runner/runner_wbtest.mbt
• tooling/test262_runner/task_selection.mbt

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📝 Walkthrough

Walkthrough

Refactors build_test_tasks error handling from try?/match to an explicit try/catch block that falls back to default_modes_for_run_mode. Switches shard string extraction in report_test262.mbt from substring calls to slice syntax. Adds two new test cases: one for the build_test_tasks fallback behavior and one for atomics short-timeout preamble normalization.

Changes

test262 runner and subprocess helper improvements

Layer / File(s)	Summary
build_test_tasks error handling and fallback test tooling/test262_runner/task_selection.mbt, tooling/test262_runner/runner_wbtest.mbt	build_test_tasks replaces try?/match with try { ... } catch { ... } that falls back to default_modes_for_run_mode(run_mode) on any error. A new test passes missing/invalid paths alongside a valid one and asserts default non-strict and strict entries are produced for all inputs.
Atomics short-timeout preamble normalization test tooling/test262_runner/runner_wbtest.mbt	Adds a test constructing HarnessOptions with atomics_timeout_profile=ATOMICS_TIMEOUT_PROFILE_SHORT and asserting the generated preamble includes NaN-handling/normalization code and bounded atomics-wait wrapper calls, while excluding a specific negative-type guard string.
Shard filename parsing: substring → slice syntax tooling/subprocess_helpers/report_test262.mbt	parse_shard_of_total extracts the left/right numeric parts of NofM using inner[:of_pos] and inner[of_pos + 2:]; validate_shard_completeness extracts the shard inner portion using slice indices derived from prefix/suffix lengths.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• dowdiness/js_engine#297: Introduced shard completeness validation and NofM filename parsing logic in report_test262.mbt that this PR directly refactors.
• dowdiness/js_engine#299: Modifies parse_shard_of_total and validate_shard_completeness in the same file and functions updated in this PR.

Poem

🐇 A slice is cleaner than a substr call,
A catch replaces match once and for all.
I hop through the harness with short-timeout care,
NaN-handling tested, with fallbacks to spare.
The shard finds its number with elegant grace —
One tiny refactor, a tidier place!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'test: harden test262 tooling regression coverage' clearly describes the main focus of the changeset: adding regression test coverage to the test262 tooling, which is the primary objective across all three modified files.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/test262-tooling-cleanups

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