profiling.sampling incorrectly handles base_frame and native frame as the first frame

Bug description

profiling.sampling does not correctly handle some frame chains whose first frame is not a Python frame.

There are two cases:

1. The first frame is base_frame

   A native thread can retain a Python thread state while waiting in C code, with no Python functions executing. Its frame chain contains only the base_frame sentinel.

   The sampler skips this sentinel and then raises:

   RuntimeError: Failed to parse initial frame in chain

2. The first frame is a native frame

   When the first frame in the chain is a native frame, the sampler does not correctly handle it as the initial frame and may report the frame chain as invalid instead of adding a <native> frame.

In both cases, the failure can cause the entire sampling call to fail, so valid Python stacks from other threads are also lost.

Reproduction

Case 1: base_frame as the first frame

Create a pthread that registers and detaches a Python thread state:

PyGILState_STATE state = PyGILState_Ensure();
PyThreadState *saved = PyEval_SaveThread();

/* Wait on a pthread condition variable while the parent samples. */

/* Cleanup after sampling. */
PyEval_RestoreThread(saved);
PyGILState_Release(state);

Keep the main thread inside a Python function and sample the child process from its parent:

unwinder = _remote_debugging.RemoteUnwinder(
    pid, all_threads=True, cache_frames=False,
)
unwinder.get_stack_trace()

tachyon-empty-native-repro.zip

• Reproduces with frame caching both enabled and disabled.

Case 2: Native frame as the first frame

Construct a frame chain whose current frame is a native frame and whose previous frame leads to base_frame.

When native-frame collection is enabled, sampling this thread currently fails to correctly handle the native frame as the first frame.

Expected behavior

The sampler should correctly handle both cases:

1. If the first frame is base_frame, treat it as a valid empty Python stack.
2. If the first frame is a native frame, add a <native> frame to the returned stack and continue walking the frame chain.

A special frame state in one thread should not cause the entire sampling operation to fail or prevent valid stacks from other threads from being returned.

Suspected cause

process_frame_chain() does not correctly handle cases where the first frame does not contain a parseable Python frame, including base_frame and native frames.

These cases can be incorrectly treated as a broken initial frame chain.

Environment

Linux, CPython 3.15.0rc2+dev free-threaded build and current main.

Linked PRs

• gh-157817

Start by tracing process_frame_chain(), focusing on how it handles base_frame and a native frame as the first frame. Use the provided pthread reproduction and the native-frame scenario to exercise both paths. Done means base_frame produces a valid empty Python stack, native frames add a frame, and one special thread does not discard valid stacks from other threads.