Improved test coverage for array exercises - FilterOddNumbers and Mean

[pali101]

This PR introduces improvement to test cases for FilterOddNumbers and Mean contracts:

1. FilterOddNumbers Contract:

• Added edge cases to handle:
  • Empty arrays
  • Arrays with all even numbers
  • Arrays with all odd numbers
  • Arrays with a single even number
  • Arrays with a single odd number
• Added array length checks to ensure the result array matches the expected size.
• Included tests for large arrays to check correctness.

2. Mean Contract:

• Re-structured tests as one function, one type of test and added comprehensive tests for the mean function.
• Covered edge cases, including:
  • Empty array (handled with Panic error check for division by zero)
  • Single-element arrays (mean should be the element itself)
  • Large numbers to ensure proper handling of large sums
• Implemented vm.expectRevert for the Panic error (0x12 for division by zero).