Test that withdraw_amount returns the requested amount

[Glittersup]

Summary

withdraw_amount returns the amount transferred, which for a successful call should equal exactly what was asked for. Nothing asserts that today, so a change that silently clamped the amount would pass.

Acceptance criteria

• A test asserts withdraw_amount returns exactly the requested amount on success.
• The assertion holds for a request smaller than the available balance.
• The suite still passes.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/tricklepay-contracts.git
cd tricklepay-contracts
git remote add upstream https://github.com/TricklePay/tricklepay-contracts.git

Create a branch for this issue:

git checkout -b test/issue-182

Suggested commit message:

test: assert the value returned by withdraw_amount

Run cargo fmt --check, cargo clippy --all-targets -- -D warnings, and cargo test before opening a pull request and linking this issue.