feat: add find_unique_number function to bit_manipulation

[AliAlimohammadi]

Description

Adds a new function find_unique_number to the bit_manipulation module that finds the unique element in an array where all other elements appear exactly twice.

Algorithm

Uses the XOR bitwise operation with the property that a ^ a = 0 and a ^ 0 = a. By XORing all elements together, paired numbers cancel out, leaving only the unique number.

Implementation Details

• Time Complexity: O(n) - single pass through the array
• Space Complexity: O(1) - constant extra space
• Uses functional approach with fold() iterator
• Includes error handling for empty input
• Comprehensive unit tests covering edge cases

Testing

• ✅ All existing tests pass
• ✅ New tests added for:
  • Basic functionality
  • Single element arrays
  • Empty arrays (error case)
  • Negative numbers
  • Large numbers
  • Zero as unique element

Checklist

• Code follows Rust style guidelines (cargo fmt)
• Passes clippy lints (cargo clippy)
• All tests pass (cargo test)
• Documentation includes examples
• Function is properly exported in module

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.71%. Comparing base (7a261d7) to head (511ab9a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #973      +/-   ##
==========================================
- Coverage   95.71%   95.71%   -0.01%     
==========================================
  Files         346      347       +1     
  Lines       22608    22642      +34     
==========================================
+ Hits        21639    21671      +32     
- Misses        969      971       +2     

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[AliAlimohammadi]

@siriak, this is ready to be merged.