Test that withdraw returns the amount transferred

[Glittersup]

Summary

withdraw returns the amount it moved, and a caller may rely on that return value rather than reading the stream again. The tests check the resulting balances but not the returned figure.

Acceptance criteria

• A test asserts the value returned by withdraw equals the amount transferred.
• The assertion holds for a partially vested stream.
• 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-181

Suggested commit message:

test: assert the value returned by withdraw

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