feat: validate expired bid cleanup semantics

[Agbasimere]

Closes: #710

Summary

Improved the robustness, accuracy, and idempotency of the cleanup_expired_bids mechanism to ensure reliable bid index hygiene and precise reporting of cleaned records.

Key Changes

• Refactored refresh_expired_bids in src/bid.rs:
  • Enhanced the cleanup loop to handle already-expired bids and orphaned bid IDs (IDs present in the index but with missing records).
  • Updated the function to return a precise count of items cleaned from the invoice's internal bid index.
  • Optimized storage writes by only updating the invoice's bid record if changes were actually made (idempotency).
• Added Comprehensive Tests:
  • test_cleanup_expired_bids_with_pre_set_expired: Verified that bids already transitioned to Expired but still in the index are correctly handled and counted.
  • test_cleanup_expired_bids_partial_idempotency: Validated partial cleanup across multiple TTL windows and confirmed 0 results on repeated calls.
  • test_cleanup_expired_bids_integration: Demonstrated correct index pruning over 7+ days of placement history.
• Fixed Infrastructure/Test Failures:
  • Resolved a syntax error (extra bracket) at the end of src/test_bid.rs.
  • Fixed missing unwrap() calls on get_invoice() results in the test suite to match the current contract API.
• Documentation:
  • Updated docs/contracts/bidding.md with a new section for the Cleanup Mechanism, detailing the public function and its return semantics.

Testing Performed

• Ran targeted unit tests: cargo test -p quicklendx-contracts test_cleanup_expired_bids
• Verified return count accuracy: Expecting 1 for a single expired bid and 0 for immediate subsequent calls.
• Validated state transitions: Confirmed Placed bids correctly transition to Expired when the TTL is exceeded.

Security Considerations

• The cleanup_expired_bids function remains public but read-and-delete only on the target invoice index, preventing manipulation of active bids.
• Bounded loops ensure that even if the MAX_BIDS_PER_INVOICE limit is reached, the cleanup operation remains gas-efficient.

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[Agbasimere]

@Baskarayelu Kindly merge