refactor: Namespace restructuring and unhappy path tests

[Kiryuumaru]

Summary

Comprehensive namespace refactoring to follow folder structure convention, plus extensive unhappy path test coverage.

Changes

Namespace Restructuring

All classes now follow the convention: folder path → namespace (omitting src/ and starting with NetConduit).

Folder	New Namespace
src/NetConduit/Enums/	NetConduit.Enums
src/NetConduit/Events/	NetConduit.Events
src/NetConduit/Exceptions/	NetConduit.Exceptions
src/NetConduit/Interfaces/	NetConduit.Interfaces
src/NetConduit/Models/	NetConduit.Models
src/NetConduit/Transits/	NetConduit.Transits

Interface Relocation

• Moved ITransit and IMessageTransit from Transits/ to Interfaces/ folder

New Tests

• Added 53 unhappy path tests for API misuse scenarios:
  • ObjectDisposedException after disposal
  • InvalidOperationException for incorrect state usage
  • NotSupportedException for unsupported operations
  • ArgumentNullException for null parameters
  • Cancellation handling with OperationCanceledException
  • Double disposal safety
  • Multiplexer disposal propagation
• Added transit eagerness tests for lazy/eager initialization patterns

Updated Imports

• Added proper using statements across all dependent files:
  • Transport packages (Tcp, Ipc, Udp, Quic, WebSocket)
  • Sample projects
  • Unit and integration tests
• Added GlobalUsings.cs to unit tests for common imports

Testing

• ✅ Build succeeds with 0 errors
• ✅ All 53 unhappy path tests pass
• ✅ Existing tests continue to pass