Test that create_stream returns the id of the new stream

[Glittersup]

Summary

create_stream returns the id it assigned, and that value is how a caller finds the stream afterwards. The tests use the returned id but never assert that it matches the stream actually stored.

Acceptance criteria

• A test asserts the id returned by create_stream identifies the stored stream.
• The assertion is made for more than one stream created in sequence.
• 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-184

Suggested commit message:

test: assert the id returned by create_stream

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