gh-151613: Fix remote debugging frame cache ABA (#151614)

The remote debugging frame cache previously used only the last_profiled_frame address as its cache anchor. If a frame returned and a later frame reused the same _PyInterpreterFrame address, the profiler could accept a stale cache entry and splice parent frames from a different call chain into the current stack.

This adds a last_profiled_frame_seq counter next to last_profiled_frame, increments it when the anchor advances, stores it in frame cache entries, and validates cache hits against both the frame address and the sequence. Cache miss walks now copy stack chunks before storing new cache entries so stored continuations come from a stable snapshot. The new regression test exercises alternating call chains and checks that cached stacks never contain frames from both branches.

Python/pystate.c

@@ -1605,6 +1605,8 @@ init_threadstate(_PyThreadStateImpl *_tstate,
     tstate->current_frame = &_tstate->base_frame;
     // base_frame pointer for profilers to validate stack unwinding
     tstate->base_frame = &_tstate->base_frame;
+    tstate->last_profiled_frame = NULL;
+    tstate->last_profiled_frame_seq = 0;
     tstate->datastack_chunk = NULL;
     tstate->datastack_top = NULL;
     tstate->datastack_limit = NULL;