pingpong: fix regression when making app calls

[cce]

Summary

Fixes a regression introduced in #6286 where constructAppTxn in the pingpong perf testing tool's code was refactored to use the new RefBundle structure for txn.Access, but forgot to set the sender output arg. This caused constructAppTxn to always return an empty string, leading to "no acct" errors for all app calls in pingpong and 0 TPS for stateful_teal_* performance tests.

The fix adds the missing sender assignments:

• Initialize sender = from at the beginning of the opt-in logic
• Update sender = from when the from address is changed to use an already opted-in account

Test Plan

Tested manually, "no acct" error no longer appears.