DataProcessor 'medain' has typo and incorrect calculation

[aliAljaffer]

As a developer
I want correct median calculation with proper spelling
So that statistical analysis is accurate

Current Behavior

1. Typo: medain() instead of median()
2. Wrong calculation for even-length lists

Location: src/java/DataProcessor.java line 40

Expected Behavior

For even-length lists, return average of two middle values.

Example

[1, 2, 3, 4] should return 2.5, not 3

Acceptance Criteria

• Rename to median()
• Fix even-length calculation
• Add unit tests for both odd and even lengths

[diptipradeep]

@aliAljaffer Could you please assign this issue to me?