[TEST] [Coinswap] Port Maker RPC Tests

[hulxv]

Description

Port all existing Maker RPC tests to the new mill-rpc implementation.

Motivation

Comprehensive testing ensures migration doesn't introduce regressions for Maker functionality.

Proposed Solution

• Offer management tests
• Swap lifecycle tests
• Error handling tests
• Concurrent swap tests
• Performance tests

Tasks

• Port unit tests
• Port integration tests
• Add mill-rpc specific tests
• Add stress tests
• Ensure comprehensive coverage

Expected

• All Maker tests pass with new implementation
• Enhanced test coverage
• Performance regression detection
• Stress testing capabilities