Fix ScopedGILRelease for non-Python threads in embedded services

Summary:
## Problem

D94096843 introduced `ScopedGILRelease` to safely release the GIL in code that may or may not be called from Python. However, the implementation only checks `PyGILState_Check()` before calling `PyEval_SaveThread()`, which is insufficient for C++ services that embed Python but also receive Thrift RPCs on non-Python threads.

On threads that were never registered with the Python interpreter (e.g., Thrift worker threads in a service that links Python for subinterpreters), `PyGILState_Check()` can return misleading results, and calling `PyEval_SaveThread()` on such a thread crashes with:

```
Fatal Python error: PyEval_SaveThread: the function must be called with the GIL held, after Python initialization and before Python finalization, but the GIL is released (the current Python thread state is NULL)
```

This affects any C++ service that links Python and receives Thrift RPCs from non-Python threads (not just limits_computer).

## Fix

Add two additional safety checks before attempting to release the GIL:

1. **`!Py_IsFinalizing()`** — Skip GIL release during Python finalization, when thread states are being torn down and GIL operations are unsafe.

2. **`PyGILState_GetThisThreadState() != nullptr`** — Verify the current thread actually has a Python thread state before checking GIL ownership. This is the key fix: non-Python threads have no thread state, so we must not call `PyGILState_Check()` or `PyEval_SaveThread()` on them.

Both `Py_IsFinalizing` and `PyGILState_GetThisThreadState` are already available as weakly-linked symbols in `folly/python/Weak.h`, so no additional symbol additions were needed.

The full guard is now:
```cpp
isLinked() && (Py_IsInitialized() != 0) && !Py_IsFinalizing() &&
(PyGILState_GetThisThreadState() != nullptr) &&
(PyGILState_Check() != 0)
```
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Reviewed By: fried

Differential Revision: D95138790

fbshipit-source-id: 83cc652ec12002fa238e7cde18912c47774c6bc0

Author: zolyfarkas-fb

third-party/folly/src/folly/python/ScopedGILRelease.h

@@ -22,8 +22,12 @@ namespace folly::python {
 
 /**
  * RAII helper to release the Python GIL for the duration of a scope.
- * Only releases if Python is linked, initialized, and the current thread holds
- * the GIL. Safe to use in code that may or may not be called from Python.
+ * Only releases if Python is linked, initialized, not finalizing, the current
+ * thread has a Python thread state, and the current thread holds the GIL.
+ * Safe to use in code that may or may not be called from Python, including
+ * non-Python threads in services that embed Python (e.g., C++ services
+ * receiving Thrift RPCs that also link Python for subinterpreters or other
+ * purposes).
  *
  * Example usage:
  *   void expensiveBlockingOperation() {
@@ -36,7 +40,8 @@ class ScopedGILRelease {
  public:
   ScopedGILRelease()
       : tState_(
-            (isLinked() && (Py_IsInitialized() != 0) &&
+            (isLinked() && (Py_IsInitialized() != 0) && !Py_IsFinalizing() &&
+             (PyGILState_GetThisThreadState() != nullptr) &&
              (PyGILState_Check() != 0))
                 ? PyEval_SaveThread()
                 : nullptr) {}