fix: preserve nested JS return expressions

[honor2030]

Summary

• Evaluate JS snippets as-is first instead of lexically scanning for return
• Retry with the existing IIFE wrapper only when Chrome reports SyntaxError: Illegal return statement
• Add regression coverage so nested function/arrow returns keep normal JavaScript completion-value semantics

Root cause

js() used a lightweight string scanner to find return before evaluation. That scanner could not distinguish top-level returns from returns nested inside functions/arrow functions, so expressions like const f = () => { return 1 }; f() were wrapped as an IIFE and lost their completion value.

Test Plan

• RED: uv run --with pytest --with-editable . python -m pytest tests/integration/test_js.py::test_nested_arrow_return_is_not_wrapped -q failed before the fix because the expression was wrapped as (function(){...})().
• GREEN: uv run --with pytest --with-editable . python -m pytest tests/integration/test_js.py -q → 18 passed
• uv run --with pytest --with-editable . python -m pytest tests -q → 95 passed
• uv run --with-editable . python -m compileall -q src tests
• uv run --with pip --with-editable . python -m pip check → No broken requirements found.
• uv run --with pip-audit --with-editable . pip-audit → No known vulnerabilities found (local package skipped because it is not on PyPI)
• uv run --with ruff --with-editable . ruff check --select B src/browser_harness/helpers.py tests/integration/test_js.py → All checks passed!

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Summary by cubic

Evaluate JS snippets as-is and only wrap them in an IIFE when Chrome reports “Illegal return statement.” This preserves completion values and stops wrapping returns inside nested functions or arrows.

• Bug Fixes
  • Remove the string-based return scanner; detect top-level return via runtime error and retry with (function(){...})().
  • Do not wrap when input already starts with ( (avoid double-wrapping).
  • Add integration tests for illegal-return retry, nested arrow returns, and whitespace after return.

^{Written for commit 987c55f. Summary will update on new commits.}

[cubic-dev-ai]

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