Use a native __getattr__ hook for AttributeError suggestions (GIL-safe re-land of the miss-only hook)

[jhonabreul]

What does this implement/fix?

Re-lands the miss-only __getattr__ hook for AttributeError member-name suggestions (the #124 approach, which #126 had replaced with a tp_getattro override), and fixes the off-GIL defect that crashed Lean's CI on 2.0.55.

Why re-land the hook: with the hook, successful attribute accesses go straight through CPython's native generic getattr with no managed transition — only an actual miss enters managed code. The #126 tp_getattro override put a managed round-trip on every attribute access, hit or miss.

The 2.0.55 crash (see first commit): the original hook exposed the hint builder to Python as a .NET delegate (Func<PyObject, string, string>). Python invoked it through DelegateObject.tp_call → MethodBinder.Invoke, and MethodBinder releases the GIL around every reflected invocation (allow_threads defaults to true). The callback therefore ran CPython C-API calls without holding the GIL on every attribute miss (hasattr, getattr with default, typos). It usually survived by luck, but segfaulted whenever the pythonnet Finalizer fired mid-callback: Finalizer.DisposeAll() starts with PyErr_Fetch, which dereferences the current thread state — NULL when the GIL is not held. This is what crashed Lean's CI test host after all 35k tests passed.

The fix (second commit): the __getattr__ installed on reflected types is now a native method descriptor — a process-lifetime PyMethodDef (METH_VARARGS) around a managed thunk, created with PyDescr_NewMethod (newly bound in Runtime.Delegates). CPython's slot_tp_getattr_hook calls the managed hook directly as a native method call with the GIL held; MethodBinder is never involved.

User-facing messages are unchanged from 2.0.55/2.0.56:

AttributeError: 'Version' object has no attribute 'majr' Did you mean: 'major'?

Does this close any currently open issues?

No open issue; this addresses the Lean CI test-host crash observed with 2.0.55 (PyErr_Fetch(tstate=NULL) under Finalizer.DisposeAll under BuildMissingAttributeMessage).

Verification performed

• Instrumented build with PyGILState_Check() inside BuildMissingAttributeMessage: the delegate-based hook (first commit alone) reports GIL held: False on every miss; with the fix it reports GIL held: True.
• 20s stress run: 6 threads doing concurrent attribute misses + CLR object churn (to keep the Finalizer queue busy) — no crash.
• Embedding tests: 909 passed. Python tests: 443 passed (one pre-existing, environment-only failure from quantconnect-stubs shadowing the Microsoft namespace, identical on master).
• Lean's Python/Pandas unit tests run against this build with the test host surviving (the 2.0.55 failure mode was a host crash after tests passed).
• Behavioral checks: hasattr/getattr-with-default, dunder probes (pickle/copy machinery), Python subclasses defining their own __getattr__, and suggestions inherited by Python subclasses of .NET types all behave as before.

A new regression test (test_missing_attribute_hook_is_native) asserts the installed __getattr__ is a native method_descriptor, the property that keeps the callback out of MethodBinder's GIL-releasing path.