Implement `multiplyStrings(a, b)` to multiply two non-negative integers given as decimal strings and return their product as a decimal string. You may NOT convert the full inputs to a built-in big-integer or fixed-width integer type, nor use any arbitrary-precision arithmetic library. Requirements: 1. Correctly multiply two non-negative integers represented as decimal strings and return the product as a string. 2. Handle edge cases: "0" (and any product that is zero), "1", inputs of very different lengths, leading zeros in the inputs, and many trailing zeros. The result must not carry spurious leading zeros (e.g. "00" should never be returned). 3. Scale efficiently to very large inputs (strings up to a few thousand digits) while managing carries correctly and avoiding quadratic memory blowups. Approach: grade-school long multiplication on digit arrays. Digit i of `a` (from the right) times digit j of `b` contributes to position i+j of the result. Accumulate every partial product di*dj into a result buffer of size n+m, then do a single carry-propagation pass, then trim leading zeros. Examples: - multiplyStrings("123", "4567") -> "561741" - multiplyStrings("0", "123") -> "0" - multiplyStrings("007", "08") -> "56" - multiplyStrings("99", "99") -> "9801"