Add tests for the per-agent fixed-window rate limiter across window rollovers

[mikewheeleer]

Test the rate-limit window math and RateLimitExceeded gate

Description

The fixed-window rate limiter in record_usage in contracts/escrow/src/lib.rs — MaxRequestsPerWindow, WindowSeconds, the RateWindow(Address) state, and the RateLimitExceeded (#15) error — has no coverage in contracts/escrow/src/test.rs. Nothing asserts that the limiter is disabled by default, that a within-window over-cap call panics #15, or that the window correctly rolls forward once now >= window_start + window_seconds. This issue closes that gap by driving the ledger clock across window boundaries.

Requirements and context

• Repository scope: Agentpay-Org/Agentpay-contracts only.
• Cover: limiter disabled when either cap or window is 0 (default) allows unbounded calls; enabled + cumulative requests exceed cap within the window → panic #15; exactly-at-cap succeeds.
• Cover: after advancing the clock past the window via env.ledger().with_mut, the count resets and recording succeeds again; the window anchors at the first in-window call.
• Cover: an agent cannot reset its own window early (recording mid-window keeps window_start fixed).
• Use #[should_panic(expected = "Error(Contract, #15)")] matching the existing test conventions; test-only change unless a genuine bug surfaces.

Suggested execution

• Fork the repo and create a branch
• git checkout -b test/contracts-rate-limit-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 rate-limit scenarios above.
  • Add documentation: note the covered behaviour in the test module header comment.
  • Include NatSpec-style doc comments (///) on any test helpers.
  • Validate security: the throttle cannot be bypassed and cannot be reset early by the agent.
• Test and commit

Test and commit

• Run cargo fmt --all -- --check, cargo build, and cargo test.
• Cover edge cases: cap reached then window rollover, single huge request exceeding cap, window length of one second, limiter half-configured (cap set, window zero).
• Include the full cargo test output and a short security notes section in the PR description.

Example commit message

test: cover fixed-window rate limiter and RateLimitExceeded gate

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]

Hi team! Per-agent fixed-window rate limiter tests. Plan: write tests that exercise the rate limiter per agent — assert requests under the limit succeed, over the limit are rejected, and the window resets correctly. Defensive Soroban coverage. Ready.