[FEATURE] [Coinswap] Port Taker RPC Tests

[hulxv]

Description

Port all existing Taker RPC tests to work with the new mill-rpc implementation.

Motivation

Comprehensive testing ensures migration doesn't introduce regressions.

Test Categories

• Unit tests for each handler
• Integration tests for full flows
• Error handling tests
• Authentication tests
• Performance tests

Tasks

• Port unit tests
• Port integration tests
• Add new mill-rpc specific tests
• Add performance regression tests
• Ensure 100% test coverage
• Add test documentation

Expected

• Full test coverage maintained
• No functionality regressions
• Improved test reliability
• Performance regression detection