Investigate issue in custom transaction web test

We had to modify the custom transaction tests in the web client to make them work. We should investigate why this change was necessary and if we can make it work as intended.

> The examples I use in both of these custom code tests are taken from different places in the repo. In this case, I tried to follow [this example](https://github.com/partylikeits1983/miden-clob-designs/blob/205ade0f7315e55a516327dfbafdd5ef7fab869a/tests/mapping_example_test.rs#L21) as closely as possible, with the exception of [deleting lines 6 & 7 of this script](https://github.com/partylikeits1983/miden-clob-designs/blob/205ade0f7315e55a516327dfbafdd5ef7fab869a/masm/scripts/mapping_example_script.masm#L6) as they were causing errors in the test and I didn't know how to fix the issues with those lines present.

_Originally posted by @dagarcia7 in [#914](https://github.com/0xMiden/miden-client/issues/914#issuecomment-2902924234)_

Look at the custom transaction tests in the web client, likely in a test directory. Compare the modified test to the original example in the linked Rust file (tests/mapping_example_test.rs) and MASM script (masm/scripts/mapping_example_script.masm). Understand why lines 6-7 of the script caused errors. Determine if the web client's transaction handling differs from the Rust example's expectations.

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