fix: isNaN() returns true for non-numeric strings instead of false

[cs01]

Summary

• isNaN("abc") now correctly returns true instead of false
• Uses strtod endptr check to detect non-parseable strings (same pattern as fix: parseInt/parseFloat return NaN for non-numeric strings #257, fix: Number() returns NaN for non-numeric strings instead of 0 #258)
• For numeric strings like "42", still does proper NaN check on the parsed value

Test plan

• npm run verify:quick passes
• New test builtins/isnan-string exercises string args, numeric strings, NaN, and numbers

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