Add ContractError stability test for streampay-stream (v7 slot 34)

[greatest0fallt1me]

Description

This is a smart-contract issue for the GrantFox campaign. This is a smart-contract issue for the GrantFox FWC26 campaign (Stellar Wave). Freeze client-facing error code numbers for streampay-stream.

Requirements and Context

• Implement per the description
• Add focused tests
• Document any API/visible changes
• Adhere to repo's lint and code style
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b task/streampay-stream-errstab-v7-34

2. Implement changes
   • contracts/contracts/streampay-stream/tests/err_stab.rs
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

test: streampay-stream err stability

Acceptance Criteria

• Implementation matches the description
• Tests added and passing
• Code review approved
• Docs updated

Guidelines

• Minimum 95% test coverage with cargo test
• require_auth on every state-changing entrypoint
• Overflow-safe math; no unwrap() in production paths
• Clear NatSpec-style /// rustdoc
• Timeframe: 96 hours

[drips-wave]

@greatest0fallt1me This issue could not be added to the Stellar Wave Program because the per-repo points budget would be exceeded (25000 / 25000 points already used). The label has been automatically removed.

The budget resets at the end of each Wave. You can review the remaining budget for your repos on the Orgs & Repos page.

ℹ️ Learn more about points budgets

[drips-wave]

@greatest0fallt1me This issue could not be added to the Stellar Wave Program because the per-repo points budget would be exceeded (25000 / 25000 points already used). The label has been automatically removed.

The budget resets at the end of each Wave. You can review the remaining budget for your repos on the Orgs & Repos page.

ℹ️ Learn more about points budgets