Add serial-port adapter, guard open/close with lock and disposal, and add tests

[c3rebro]

Motivation

• Make serial transport testable by removing direct dependency on System.IO.Ports.SerialPort and enable unit testing of open/close behavior.
• Harden serial port handling to avoid races and improper use after disposal when opening/closing concurrently.
• Ensure buffers are discarded and event handlers are cleaned up on disconnect/dispose to avoid resource leaks.

Description

• Introduce ISerialPortAdapter and SerialPortAdapter in src/Drivers/Transport/SerialPortAdapter.cs to wrap SerialPort for testability.
• Refactor SerialPortTransport to depend on ISerialPortAdapter, add a SemaphoreSlim _connectionLock, _disposed checks, ThrowIfDisposed() and CreateConfiguredPort() factory method, and ensure event unsubscription and buffer discard on dispose.
• Replace ad-hoc Task.Run wrappers around open/close with proper async locking and disposal guards in ConnectAsync/DisconnectAsync and add safe Dispose logic.
• Add unit tests tests/Elatec.NET.Tests/SerialPortTransportTests.cs with a FakeSerialPortAdapter to validate open-once behavior, disconnect buffer discard, dispose closing, and thrown exceptions when using after dispose.

Testing

• Ran dotnet test tests/Elatec.NET.Tests/Elatec.NET.Tests.csproj to execute the test suite.
• All tests passed: Passed! - Failed: 0, Passed: 21, Skipped: 0, Total: 21.
• Test run produced some non-fatal compiler warnings about unused events, but no test failures occurred.
• Unit tests cover ConnectAsync, DisconnectAsync, Dispose, and behavior when calling ConnectAsync after disposal.

---

Codex Task