test(bidding): add cancel-vs-accept interleaving regression

[Kuhai9801]

📝 Description

Adds focused Soroban regression coverage for cancel_bid versus accept_bid_and_fund on the same bid.

🔧 Changes Made

• Adds quicklendx-contracts/src/test_bid_cancel_accept_race.rs.
• Covers cancel-then-accept: the later accept fails with InvalidStatus, no escrow/investment/funding state is created, and the cancelled bid is not returned by get_best_bid.
• Covers accept-then-cancel: the later cancel returns false, while bid, invoice, escrow, investment, indexes, and token balances stay consistent.

🧪 Testing

• rustfmt --check --edition 2021 quicklendx-contracts/src/test_bid_cancel_accept_race.rs
• git diff --check
• Not run locally: cargo test / cargo clippy

🔗 Related Issues

Closes #1302

🧪 How to Test

1. cd quicklendx-contracts
2. cargo test --features legacy-tests test_bid_cancel_accept_race -- --nocapture

[Kuhai9801]

Ready for review.

One acceptance-note caveat: the issue asks for the losing operation to fail with QuickLendXError. The cancel-then-accept ordering does assert QuickLendXError::InvalidStatus. For the accept-then-cancel ordering, the existing public cancel_bid API returns bool, so this PR documents and asserts the current guard behavior: cancel_bid returns false once the bid is already Accepted, without mutating funded invoice, escrow, or investment state.

I kept this test-only rather than changing the public contract API.

CI is green on the latest push. The repository workflow runs cargo check, cargo test, WASM size checks, and coverage gates. It does not currently run cargo clippy; a whole-repo clippy run still hits pre-existing repo-wide lints outside this PR’s scoped changes.

[Baskarayelu]

good interleaving regression - cancel racing accept on the same invoice is precisely the concurrency edge worth pinning. merging 👍