Function allocate_port 

pub fn allocate_port(
    repo_id: &str,
    branch: &str,
    _session_uuid: &str,
    port_range_start: u16,
    port_range_end: u16,
    existing_leases: &HashMap<String, PortLease>,
) -> Result<u16, WorktreeError>

Allocate a port for a branch by probing from preferred port, wrapping around.

Returns the allocated port number.