create_stream rustdoc Errors omits InvalidRate

[akprinciple]

docs: Add InvalidRate to create_stream # Errors list

closes #797
This PR addresses a documentation inconsistency in the create_stream function. The function can return StreamError::InvalidRate when the calculated rate per second truncates to zero, but this error was missing from the function's doc comment in the # Errors section.

This change updates the documentation to include InvalidRate, ensuring that developers are aware of all possible error conditions when using the function.

Why this matters

The # Errors section for create_stream was out of sync with the implementation. It was missing the InvalidRate error, which can occur when net_amount / duration truncates to 0. Accurate documentation is crucial for contract usability and helps prevent integration errors.

Acceptance Criteria

• The InvalidRate error is added to the # Errors list in the create_stream function's doc comment in contracts/stream_contract/src/lib.rs.

Out of Scope

• Updating the # Errors sections for any other functions.

[ogazboiz]

heads up: main's ci was broken (the Backend CI and Backend Docker Image CI jobs) until the fixes in #969 and #974 just landed, so the red backend/docker checks on this pr are almost certainly stale, they ran against the broken main. please rebase to re-test against the now-green main: git fetch origin && git rebase origin/main && git push --force-with-lease. once it's green i'll review and merge. (if a non-backend check like Frontend CI is still red after the rebase, that part is a real issue worth a look, since frontend ci was passing on main.)

[akprinciple]

@ogazboiz Kindly check now boss