Add test coverage for compute_route_fee state mutations: counter, timestamp, and route event

[mikewheeleer]

Test the routing side effects of compute_route_fee

Description

compute_route_fee in src/lib.rs has four side effects on a successful call: it increments TotalRoutesAllTime, stamps PairLastRouteAt, and publishes the route event with (source, destination, amount). The existing tests check the returned fee and the counter once, but never assert the route event payload, never verify the timestamp is stamped from a non-zero env.ledger().set_timestamp, and never confirm the counter is shared across multiple distinct pairs. This issue adds focused coverage of the mutation surface.

Requirements and context

• Repository scope: StableRoute-Org/Stableroute-contracts only.
• Cover: route event topic route and data (source, destination, amount) via env.events().all().
• Cover: PairLastRouteAt reflects a custom env.ledger().set_timestamp value after a route, and is None before.
• Cover: TotalRoutesAllTime increments once per route and is global across two different pairs.
• Cover: quote_route does NOT emit a route event nor change the counter (parity guard) — assert event count unchanged.

Suggested execution

• Fork the repo and create a branch
• git checkout -b test/contracts-39-route-side-effect-tests
• Implement changes
  • Write code in: src/lib.rs — no production change expected (open a follow-up if a gap is found).
  • Write comprehensive tests in: src/lib.rs #[cfg(test)] mod test — the side-effect suite described above.
  • Add documentation: note the routing side-effect test matrix in README.md or docs/testing.md.
  • Include NatSpec-style doc comments (///) on any new test helper.
  • Validate security assumptions: every documented side effect is asserted; quote_route proven non-mutating.
• Test and commit

Test and commit

• Run cargo fmt --all -- --check, cargo build, and cargo test.
• Cover edge cases and failure paths: event payload, timestamp before/after, global counter across pairs, quote non-mutation.
• Include the full cargo test output and a short security notes section in the PR description (threat model + mitigations).

Example commit message

test: cover compute_route_fee counter, timestamp, and route event

Guidelines

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

Community & contribution rewards

• 💬 Join the StableRoute community on Discord for questions, reviews, and faster merges: https://discord.gg/37aCpusvx
• ⭐ 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.

[Baskarayelu]

I’d love to be assigned this issue and contribute across the relevant areas, whether it involves smart contracts, UI development, backend logic, design implementation, or overall system integration. I’m comfortable working across the full stack and can adapt to the project’s technical requirements to ensure a smooth and consistent contribution.