Add test coverage proving compute_route_fee debits PairLiquidity and emits liq_used only when liquidity is bounded

[mikewheeleer]

Test the liquidity-consumption branch and the i128::MAX sentinel skip

Description

compute_route_fee in src/lib.rs debits amount from PairLiquidity via saturating_sub and emits a liq_used event only when the stored liquidity is not the unbounded i128::MAX sentinel — when no oracle has set liquidity, the decrement and event are skipped entirely. This dual behaviour (debit-and-emit vs skip) is core to the routing model but is not directly asserted in the existing test modules. This issue adds focused coverage for both branches and the saturating-sub edge at exact-balance and over-balance.

Requirements and context

• Repository scope: StableRoute-Org/Stableroute-contracts only.
• Test that, with liquidity set, a route debits exactly amount and get_pair_liquidity reflects the remainder.
• Test that a liq_used event is emitted carrying (source, destination, remaining) only in the bounded case, and not emitted when liquidity is the unset sentinel.
• Test the exact-balance route (remaining becomes 0) and confirm a subsequent route fails with InsufficientLiquidity.
• This is distinct from the issues-1 "decrement reported pair liquidity" feature issue — that adds the behaviour; this issue verifies it with tests.

Suggested execution

• Fork the repo and create a branch
• git checkout -b test/contracts-liquidity-consumption-coverage
• Implement changes
  • Write code in: src/lib.rs — no production change expected; if a gap is found, note it for a follow-up.
  • Write comprehensive tests in: src/lib.rs (inline #[cfg(test)] mod test, near mod test_i15_bounds_liquidity) — cover the bounded debit, the sentinel skip, the exact-balance drain, and event presence/absence via env.events().all().
  • Add documentation: none required beyond test doc comments.
  • Include NatSpec-style doc comments (///) on each new test explaining the branch it pins, matching the existing style in lib.rs.
  • Validate security assumptions: liquidity can never go negative; the sentinel is never decremented.
• Test and commit

Test and commit

• Run cargo fmt --all -- --check, cargo build, and cargo test.
• Cover edge cases: bounded debit, sentinel skip, exact drain to zero, and over-balance rejection.
• Include the full cargo test output in the PR description.

Example commit message

test: cover compute_route_fee liquidity debit, sentinel skip, and liq_used 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.

[gloskull]

Hi, I’d love to work on this issue. I have 4 years of experience across backend, frontend, full-stack, and smart contract development. I understand the task clearly and can deliver a clean, efficient, well-tested solution on time, with strong focus on code quality and maintainability. My PR will be submitted in under 10 hours.