Validate selector support at run time.

[ioquatix]

While io_uring, epoll, and kqueue may be available at compile time, their actual support at runtime is not guaranteed. Various factors, such as kernel configuration, security policies (seccomp, AppArmor, SELinux), or resource limits, may prevent them from working as expected.

This PR introduces explicit runtime checks for each event selector, ensuring that we only define the corresponding constant when the selector is truly available.

While it would be possible to defer these checks until selector creation, this approach follows the existing pattern of defining constants at initialization time, keeping the implementation consistent with the current interface.

Changes

• Adds IO_Event_Selector_URing_supported_p(), IO_Event_Selector_EPoll_supported_p(), and IO_Event_Selector_KQueue_supported_p(), which attempt to create a minimal instance of each selector to verify availability.
• Updates Init_IO_Event_Selector_*() to only define the corresponding selector if the runtime check passes.
• Ensures that systems where a selector is unavailable fail gracefully without requiring conditional compilation.

Why?

• Prevents runtime failures when attempting to use an unavailable selector.
• Improves compatibility with sandboxed environments, containers, and restricted runtime configurations.
• Aligns with existing interface patterns by conditionally defining support constants.

Types of Changes

• Bug fix.
• New feature.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

Example output:

samuel@aiko ~/D/s/io-event (io_uring-supported-runtime)> sudo sysctl -w kernel.io_uring_disabled=2
kernel.io_uring_disabled = 2
samuel@aiko ~/D/s/io-event (io_uring-supported-runtime)> bundle exec sus
/home/samuel/Developer/socketry/io-event/ext/IO_Event.so: warning: Init_IO_Event_Selector_URing: io_uring_queue_init() was available at compile time but failed at run time: Invalid argument

145 passed 2 skipped out of 147 total (1098 assertions)
🏁 Finished in 162.5ms; 6755.756 assertions per second.
🤩 Your test suite is amazing!
🎉 Great job! Your test suite has excellent performance (6755.8 < 10000)!
🐇 No slow tests found! Well done!
samuel@aiko ~/D/s/io-event (io_uring-supported-runtime)> sudo sysctl -w kernel.io_uring_disabled=0
kernel.io_uring_disabled = 0
samuel@aiko ~/D/s/io-event (io_uring-supported-runtime)> bundle exec sus
194 passed 2 skipped out of 196 total (1230 assertions)
🏁 Finished in 203.9ms; 6032.116 assertions per second.
🤩 Your test suite is amazing!
🎉 Great job! Your test suite has excellent performance (6032.1 < 10000)!
🐇 No slow tests found! Well done!
samuel@aiko ~/D/s/io-event (io_uring-supported-runtime)>