Function validate_input 

pub fn validate_input(
    tasks: &[Task],
    resources: &[Resource],
) -> ValidationResult

Validates the input data for a scheduling problem.

Checks:

1. No duplicate task IDs
2. No duplicate activity IDs (across all tasks)
3. No duplicate resource IDs
4. All tasks have at least one activity
5. All resource references in activities point to existing resources
6. All predecessor references point to existing activities
7. No circular precedence dependencies

Returns

Ok(()) if all checks pass, Err(errors) with all detected issues.