Replace use of math operators with strict_ fns

[leighmcculloch]

Remove use of math operators and use fns like checked_add, strict_add, etc.

Why: encourage devs to think about overflow failure cases.

Offline discussion: https://stellarfoundation.slack.com/archives/C030Z9EHVQE/p1762564767033219?thread_ts=1762484881.133289&channel=C030Z9EHVQE&message_ts=1762564767.033219

[leocagli]

Hi, I'd like to replace the raw math operators in the contract examples with their strict_ equivalents from soroban-sdk. Could you assign me?