Function validate_begin_num 

pub fn validate_begin_num(
    begin: i32,
    num: i32,
    max_num: i32,
    endpoint: &str,
) -> Result<PageBounds, PageBoundsError>

Validate (begin, num) pagination parameters.

Rules:

• begin >= 0
• 0 <= num <= max_num

num = 0 is intentionally valid and represents an empty page request, which matches the audited C++/backend behavior for the affected endpoints.