fix: restore v4 numeric-string + number addition behavior

[SergioChan]

Summary

• restore backward-compatible + behavior for mixed string/number operands when the string parses as numeric
• keep existing string concatenation behavior when the string side is non-numeric
• add regression tests for "10" + 5, 5 + "10", and non-numeric fallback concatenation

Testing

• Not run locally: go toolchain is unavailable in this runtime (go: command not found), so go test ./... could not be executed here.
• Static validation run: git diff --check (no whitespace issues).

Related

Fixes #342

[sonarqubecloud]

Quality Gate failed

Failed conditions
3.6% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud