Refactor mavconn

[vooon]

Summary

This PR modernizes and hardens libmavconn threading/lifecycle behavior, improves URL parsing validation, and adds initial CI coverage reporting.

What Changed

• Refactored transport IO lifecycle with shared/owned io_service support and a common runner.
• Fixed close-path/thread lifecycle edge cases (including callback-thread shutdown).
• Reduced lock scope and removed recursive mutex usage in transport paths.
• Modernized code to C++20 style in touched areas (std::jthread, lambdas, [[nodiscard]] where appropriate).
• Hardened URL parsing with strict numeric/range validation for ports and ids.
• Expanded tests for:
  • shared-io threading behavior,
  • callback-close safety,
  • invalid URL parse cases.
• Added coverage.yml workflow with an extensible component matrix (enabled for libmavconn first).

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