test(backend/copilot): add regression tests for 3 pre-launch bugs

Reproduce the three production bugs that the pre-launch speculative
execution mechanism caused, verifying the current direct-execution
handler is free of them:

Bug 1 (SECRT-2204): Duplicate execution on arg mismatch
  - test_single_execution_even_with_different_arg_representations
  - test_concurrent_calls_each_execute_once

Bug 2: FIFO desync when security hook denies a tool
  - test_skipped_call_does_not_affect_subsequent_calls
  - test_handler_has_no_shared_queue_state

Bug 3: Cancel race condition (task completes before cancel)
  - test_no_speculative_execution_before_handler_called
  - test_failed_execution_does_not_leave_orphaned_tasks