Fix the type error of client example

[minggangw]

#1030 changed to leverage BigInt to represent int64, which leads to the type error of the client example. This PR fixes the type error by updating the client example to use BigInt instead of number.

Fix: #1145

[coveralls]

coverage: 84.763%. remained the same
when pulling 7239bee on minggangw:fix-exmaple-error
into 4405557 on RobotWebTools:develop.

[Copilot]

Pull Request Overview

This PR updates the client example to ensure a and b are passed as BigInt values, matching the service’s expected input types.

• Converted a and b from Number to BigInt
• Preserved random range logic while addressing the type error