Add regression tests for issues identified during the Codex review

[vshalygin]

This testing issue was identified during a Codex code review.

Add focused regression and contract tests for the uncovered cases found during the review:

• move construction and move assignment of a default thread_pool_task;
• conversion of unique_ptr2<Derived> to a base type without a virtual destructor, where appropriate as a documented contract test;
• stopping and destroying a thread pool from one of its worker threads, where testable without terminating the main test process;
• construction of basic_thread_pool with zero threads;
• iteration, reverse iteration, and iterator subtraction for empty buffer, buffer_view, and cbuffer_view;
• compile-time iterator-concept checks and representative standard algorithms;
• null function pointers, null member-function pointers, and empty std::function targets in cl::function;
• lvalue and move-only callbacks passed to multiple_timer;
• periodic_timer with zero periods;
• all empty buffer construction and copy paths;
• actual callback exception handling in periodic_timer;
• direct behavioral tests for event and spinlock.

The existing PeriodicTimer.CatchesExceptionInCallback test should also be corrected because it currently does not configure the callback to throw.