feat: add idempotency on the place_bid entrypoint

[AugistineCreates]

This pr closes #1505

This PR introduces idempotency for the place_bid entrypoint.

Description

• Required salt (32-byte BytesN<32>) argument is added to place_bid.
• The contract checks the unique combination of (invoice_id, investor, salt).
• If a duplicate bid is submitted, the contract returns ContractError::DuplicateBid (error code 2204) instead of accepting it again.
• Includes a unit test to verify the duplicate bid rejection behavior.

Threat Mitigated

Mitigates the threat of replay or double-submission of bids, preventing duplicate resource allocation and race conditions on the contract.

[drips-wave]

@AugistineCreates Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits