Add zero-division checks to stdMath.percentDelta overloads (#735)

### PR Description
- **What**: Add explicit zero-divisor guards to
`stdMath.percentDelta(uint256,uint256)` and
`stdMath.percentDelta(int256,int256)`.
- **Why**: Prevents runtime division-by-zero and makes failure mode
explicit and predictable.
- **How**: Added `require` checks with a concise error message and brief
comments.

---------

Co-authored-by: zerosnacks <[email protected]>
Co-authored-by: zerosnacks <[email protected]>

Author: viktorking7

src/StdMath.sol

@@ -29,6 +29,8 @@ library stdMath {
     }
 
     function percentDelta(uint256 a, uint256 b) internal pure returns (uint256) {
+        // Prevent division by zero
+        require(b != 0, "stdMath percentDelta(uint256,uint256): Divisor is zero");
         uint256 absDelta = delta(a, b);
 
         return absDelta * 1e18 / b;
@@ -37,6 +39,8 @@ library stdMath {
     function percentDelta(int256 a, int256 b) internal pure returns (uint256) {
         uint256 absDelta = delta(a, b);
         uint256 absB = abs(b);
+        // Prevent division by zero
+        require(absB != 0, "stdMath percentDelta(int256,int256): Divisor is zero");
 
         return absDelta * 1e18 / absB;
     }

test/StdMath.t.sol

@@ -125,7 +125,7 @@ contract StdMathTest is Test {
         assertEq(stdMath.percentDelta(5000, uint256(2500)), 1e18);
         assertEq(stdMath.percentDelta(7500, uint256(2500)), 2e18);
 
-        vm.expectRevert(stdError.divisionError);
+        vm.expectRevert("stdMath percentDelta(uint256,uint256): Divisor is zero");
         stdMathMock.exposed_percentDelta(uint256(1), 0);
     }
 
@@ -163,7 +163,7 @@ contract StdMathTest is Test {
         assertEq(stdMath.percentDelta(5000, int256(2500)), 1e18);
         assertEq(stdMath.percentDelta(7500, int256(2500)), 2e18);
 
-        vm.expectRevert(stdError.divisionError);
+        vm.expectRevert("stdMath percentDelta(int256,int256): Divisor is zero");
         stdMathMock.exposed_percentDelta(int256(1), 0);
     }