This PR fixes memory safety and interrupt handling issues discovered while debugging IO#close across fibers.

Fix interrupt handling affecting rb_io_blocking_operation_exit

If rb_io_blocking_operation_exit executes with pending interrupts, user code that checks interrupts can exit unexpectedly. Instead, move the exception check into the EC_PUSH_TAG block so that by the time we reach rb_io_blocking_operation_exit the exception is already propagating and it behaves more like ensure.

Without this change, the following program can hang:

#!/usr/bin/env ruby

require_relative 'lib/async'

Async do
  r, w = IO.pipe
  
  read_thread = Thread.new do
    Thread.current.report_on_exception = false
    r.read(5)
  end
  
  # Wait until read_thread blocks on I/O
  Thread.pass until read_thread.status == "sleep"
  
  close_task = Async do
    r.close
  end
  
  close_task.wait
  begin
    read_thread.join
  rescue => error
    puts "Caught exception: #{error.class} - #{error.message}"
  end
end

This is a follow up to #12839

Fix use-after-free in blocking_operation_wait hook

IO#close could trigger stack-allocated memory access after the function returned. The new implementation uses a heap-allocated BlockingOperation instance which also exposes a C interface for (safe) native work pool execution.

https://bugs.ruby-lang.org/issues/21198

Technical Implementation

The blocking operation system now provides:

• Thread-safe cancellation with atomic state transitions
• Cross-platform atomic operations supporting Windows, macOS, and Linux
• Anonymous BlockingOperation class hidden from public API
• Comprehensive state machine for operation lifecycle management
• GC-safe object management with proper root registration

API Improvements

• rb_fiber_scheduler_blocking_operation_extract - Safely extract operation data while holding GVL
• rb_fiber_scheduler_blocking_operation_execute - Execute in thread pools without GVL concerns
• rb_fiber_scheduler_blocking_operation_cancel - Thread-safe cancellation support
• Consistent naming with RB_FIBER_SCHEDULER_BLOCKING_OPERATION_STATUS_* enum values

Testing

Enhanced tests covering:

• IO#close across fibers
• Memory safety verification
• Blocking operation cancellation
• Interrupt handling scenarios