[ace6tao2] Linux Proactor Improvements

[simpsont-oci]

A few things:

• Improve general test support / coverage for Proactor implementations (backends) by introducing common Proactor_Test_Backend parsing and instantiation support and having all Proactor tests make use of it.
• Add Proactor Stress test and general API contract test
• Fix several stability and correctness issues in POSIX Proactor implementations exposed by new test coverage
• Add new Linux io_uring Proactor implementation (currently seems to outperform all POSIX Proactors on Linux, especially in the new stress test).

Summary by CodeRabbit

Release Notes

• New Features

  • Added optional Linux io_uring-based asynchronous I/O backend for improved performance on modern kernels.

• Bug Fixes

  • Improved proactor stability and resource cleanup across all backends.
  • Enhanced socket handle management in connect operations.
  • Fixed transmit-file and datagram operation handling.

• Improvements

  • Expanded proactor test coverage across all available backends.
  • Added comprehensive contract testing for async I/O operations.
  • Better IPv6 support in datagram operations.