refactor: always use multiprocess spawn

[hfudev]

closes #294

[Copilot]

Pull Request Overview

This PR refactors the multiprocessing implementation to always use the spawn context instead of platform-specific contexts, which addresses stability issues and ensures consistent behavior across platforms.

• Replaces platform-specific multiprocessing context selection with unified spawn context
• Consolidates MessageQueue implementation and moves it to the main module with proper manager support
• Removes unstable test marker and updates fixture dependencies

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pytest-embedded/__init__.py	Introduces centralized MP_CTX using spawn context and consolidated MessageQueue/MessageQueueManager classes
pytest-embedded/plugin.py	Updates fixture to use new MessageQueueManager and adds proper MessageQueue cleanup handling
pytest-embedded/log.py	Removes old MessageQueue implementation and platform-specific context, imports from main module
pytest-embedded/dut_factory.py	Updates imports and removes old context/queue generator functions
pytest-embedded/dut.py	Updates type hint to use proper MessageQueue type
tests/test_base.py	Removes xfail marker from previously unstable test

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[hfudev]

Tested locally on macOS, all tests are passing.

On Windows, tests are still failing (also failing with the current 1.17.0 version).
I'll merge this PR first.