fix: avoid retrying user JavaScript errors

[ShiroKSH]

Summary

• Retry a JavaScript snippet only for Chrome's exact top-level-return parser error.
• Avoid a second evaluation after user code throws an error containing the same text.
• Add regressions for user-thrown Error and SyntaxError values.

Root cause

The retry decision used a substring of a flattened runtime error. User code could throw that text after changing page state, causing the harness to execute the expression twice.

Checks

• uv run --with pytest pytest tests/integration/test_js.py tests/unit/test_helpers.py -q
• git diff --check

---

Summary by cubic

Only retry JavaScript evaluation for Chrome’s exact top‑level return SyntaxError. This prevents double execution and side effects when user code throws errors that mention “Illegal return statement”.

• Bug Fixes
  • Detect Chrome’s top‑level return via result fields and raise _IllegalTopLevelReturn; helpers.js retries once by wrapping the expression.
  • Remove substring-based matching; do not retry on user‑thrown Error/SyntaxError that include “Illegal return statement”.
  • Add tests covering Chrome’s error path and regressions for user‑thrown errors.

^{Written for commit 182783e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[Kastan97]

Ollie (CTO) Review: APPROVED ✅

What this does: Correctly identifies Illegal return statement via CDP's className (SyntaxError) instead of fragile string matching on the error message. Prevents retrying genuine user throw errors that happen to mention 'Illegal return' in their message.

Good to merge once CI passes.