Negative test: unauthorized settle or withdraw

[greatest0fallt1me]

Description

If settle/withdraw requires auth in final design, add tests ensuring wrong signer cannot drain.

Requirements and context

• Align with token integration.
• Use granular auth testing patterns.

Suggested execution

• Fork the repository and create a branch (example below).
• Implement the changes in the StreamPay-Contracts Soroban crate (streampay-contracts).
• Keep the contract secure, tested, and documented; prefer small, reviewable diffs.

git checkout -b test/unauthorized-settle-withdraw

Implementation targets

• src/lib.rs tests without mock_all_auths where applicable.

Tests and validation

• Document Soroban auth testing pattern.

• Run cargo test in StreamPay-Contracts.

• Cover edge cases; capture test output and short security notes in the PR description.

Guidelines

• Target minimum 95% test coverage for touched contract code (or justify gaps explicitly).
• Clear documentation (rustdoc / project docs as specified).
• Timeframe: 96 hours from assignment.

Example commit message

test(contracts): unauthorized payout attempts fail

[icodeBisola]

@icodeBisola has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

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