Expand integration test suite

[Folyd]

Add more comprehensive integration tests covering real-world scenarios and edge cases.

Tasks

• Add multi-party communication tests (3+ participants)
• Add tests for all message routing scenarios:
  • Direct mode
  • Nested mode
  • Routed mode with multiple hops
  • Mixed mode scenarios
• Add error recovery tests:
  • Connection failures and retries
  • Partial message delivery
  • Timeout handling
• Add protocol compliance tests:
  • TSP specification conformance
  • DID method compliance
  • CESR format compliance
• Add state transition tests:
  • Relationship lifecycle
  • Key rotation scenarios
  • VID updates
• Add concurrency tests:
  • Simultaneous messages
  • Concurrent VID operations
  • Race condition testing
• Add cross-feature tests (different feature flag combinations)

Acceptance Criteria

• Integration test coverage increases by >30%
• All major use cases have end-to-end tests
• Edge cases and error scenarios are tested
• Tests verify protocol compliance

Priority

High - Ensures real-world reliability

Part of #245