[BOUNTY] Expand Contract Integration Test Coverage

[sayanget]

Overview

This PR significantly expands the integration and stress test coverage for the PrivacyLayer Soroban contract, fulfilling the requirements of the [Expand Contract Integration Test Coverage] bounty.

New Test Coverage

• Denomination Logic: Added tests to verify correct stroop/microunit amounts for all supported XLM and USDC denominations.
• Stress Testing: Implemented sequential deposit stress tests (up to 50 deposits) to verify the incremental Merkle tree logic and filled subtree updates under heavy state changes.
• Edge Cases:
  • Verified that uninitialized contracts correctly reject deposits.
  • Verified that zero-commitments are always rejected.
  • Verified that insufficient funds lead to expected contract errors.
  • Added tests for root history circular buffer overflow and eviction.
• Admin Operations: Expanded tests for unauthorized admin access and successful verification key updates.

Performance & Reliability

• All tests are designed to run within the Soroban test environment with mock_all_auths and budget().reset_unlimited() to ensure cryptographic consistency with Poseidon2 hashes.
• Successfully passed all 41 unit tests (including 5 new comprehensive stress tests).

[sayanget]

Claim Information

• Bounty Task: [BOUNTY] Expand Contract Integration Test Coverage
• Estimated ROI: 00+ USDC
• Payment Address (RTC): RTC89b248bde16082e0b7b9cb1b0ab29aa361a962c0