Class PhoneNumberGenerator

src/format/format.rs:120–132

First, let's modify the PhoneNumberGenerator struct to add the infix field

// First, let's modify the PhoneNumberGenerator struct to add the infix field
pub struct PhoneNumberGenerator {
    country_code: String,                    // Country calling code (e.g., "1" for US, "65" for SG)
    selected_area_codes: Vec<String>,        // Selected area codes based on prefix filter
    digits_per_number: usize,                // Number of digits to generate after country+area code
    prefix: Option<String>,                  // User-specified prefix (may override or extend area code)
    suffix_filter: Option<String>,           // Numbers must end with this suffix
    infix_filter: Option<String>,            // Numbers must have this infix at specific position from the end
    current_area_code_idx: usize,            // Current area code index
    current_number: u64,                     // Current number in sequence
    max_numbers_per_segment: u64,            // Maximum numbers per segment
    has_more: bool,                          // Whether more numbers can be generated
    remaining_area_code_parts: Vec<String>,  // Remaining parts of area codes after partial prefix match
}

// Now modify the new() method to accept an infix parameter
impl PhoneNumberGenerator {