Add tests for compute_billing across free, priced, and saturated combinations

[mikewheeleer]

Test compute_billing's multiplication and zero/saturation behaviour

Description

compute_billing in contracts/escrow/src/lib.rs returns accumulated_requests * price_per_request with saturating_mul, returns 0 when either side is zero, and is the read-only mirror of the math inside settle. contracts/escrow/src/test.rs does not directly test compute_billing across its cases — zero usage, zero price (free service), a normal product, and the saturation edge. This issue adds focused coverage and verifies compute_billing agrees with the billed value settle would return for the same state.

Requirements and context

• Repository scope: Agentpay-Org/Agentpay-contracts only.
• Cover: zero usage → 0; zero price (free service) → 0; a normal requests * price product computed correctly; an unpriced and unused pair → 0.
• Cover the saturation edge: a large requests × large price saturates at i128::MAX rather than overflowing.
• Cover that compute_billing(agent, svc) equals the value settle returns for the same pre-settle state (drive the state, read compute_billing, then settle and compare).
• Test-only change unless a genuine bug surfaces.

Suggested execution

• Fork the repo and create a branch
• git checkout -b test/contracts-compute-billing-tests
• Implement changes
  • Write code in: contracts/escrow/src/lib.rs — no changes expected; only touch if a bug is found.
  • Write comprehensive tests in: contracts/escrow/src/test.rs — the zero, free, product, saturation, and settle-agreement scenarios above.
  • Add documentation: note the covered billing math in the test module header comment.
  • Include NatSpec-style doc comments (///) on any test helpers.
  • Validate security: billing math never overflows and the read mirrors settlement.
• Test and commit

Test and commit

• Run cargo fmt --all -- --check, cargo build, and cargo test.
• Cover edge cases: zero usage, zero price, large product, saturated product, compute-vs-settle equality.
• Include the full cargo test output and a short security notes section in the PR description.

Example commit message

test: cover compute_billing free, priced, and saturated cases

Guidelines

• Minimum 95 percent test coverage for impacted modules.
• Clear, reviewer-focused documentation.
• Timeframe: 96 hours.

Community & contribution rewards

• 💬 Join the AgentPay community on Discord for questions, reviews, and faster merges: https://discord.gg/eXvRKkgcv
• ⭐ This is a GrantFox OSS / Official Campaign task and may be rewarded. When your PR is merged you'll be prompted to rate the project — if this issue and the maintainers helped you ship, we'd be grateful for a 5-star rating. Clear questions in Discord and tidy, well-tested PRs are the fastest path to a merge and a reward.

[gloskull]

Hello, I have 4 years of experience delivering solutions for projects like this, covering backend, frontend, and smart contract development. After carefully reviewing your requirements, I have a clear understanding of the task and am confident in my ability to deliver a clean, efficient, and thoroughly tested solution on time. I prioritize code quality, maintainability, and industry best practices throughout the development process.
My PR will be submitted in under 10hours.

[GBOYEE]

Hello! I can work on this. I'll implement the solution with proper Soroban patterns and comprehensive test coverage. PR incoming shortly.

[Yerimahjr]

I have extensive experience writing Soroban contract tests in Rust across multiple GrantFox repos (recently merged fuzz tests for JointSave and security tests for Xelma-Blockchain). For this issue I'll add focused tests in contracts/escrow/src/test.rs covering: zero usage → 0, zero price (free service) → 0, normal product computed correctly, unpriced and unused pair → 0, saturation edge (i128::MAX rather than overflow), and compute_billing agreement with settle for the same pre-settle state. Test-only change unless a genuine bug surfaces.

[grantfox-oss]

🦊 GrantFox — @Yerimahjr has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #115)
2. Your PR will be reviewed by the Agentpay-Org maintainers

Good luck! Track your progress on GrantFox.