Test cases - implementation needed

[MarkosComK]

Description

Current test suite lacks coverage for several important scenarios and edge cases across multiple functions.

Areas Needing Coverage

Character Functions

• Test non-ASCII characters in character manipulation functions
• Add overflow testing for toLower/toUpper edge cases
• Test character functions with extended ASCII values (128-255)

String Operations

• Add tests for strings containing null bytes
• Test concat with buffer overflow scenarios
• Test compare with strings of different lengths
• Add boundary testing for contains with overlapping patterns
• Test findR with repeated characters

Math Operations

• Add overflow testing for pow function
• Test sqrt with maximum uint64 values
• Add negative number handling tests for abs
• Test fibonacci with boundary conditions
• Test factorial with edge cases

Cast Functions

• Add tests for atoi with:
  • Multiple leading zeros
  • Maximum/minimum integer values
  • Invalid number formats
  • Mixed valid/invalid characters

Implementation Notes

• Each test case should follow existing test style in char_test.zig
• Use printTestResult helper for consistent output formatting
• Follow Zig testing best practices