This PR fixes a missed optimization.

The function_Py_CallBuiltinClass_StackRefSteal usesSTACKREFS_TO_PYOBJECTS. However, the code was not passing thePY_VECTORCALL_ARGUMENTS_OFFSET flag to the callee. This forced vector-callable types to reallocate and copy the arguments whenever they needed to prepend an argument.

Verification
Using LLDB to inspectlong_vectorcall when reached via this path:

• Before fix:nargsf = 1
• After fix:nargsf = 0x8000000000000001 (High bit correctly set)

Benchmarks
I ran a small benchmark file allocating a class in a tight loop to trigger the path:

• Baseline: 1.6193s
• Optimized: 1.5560s
• Result: ~3.9% speedup in this specific specialized call path.

gh-143361

importtimeclassWorker:def__init__(self,a,b,c):self.a=aself.b=bself.c=cdefrun_benchmark(iterations):cls=Workerstart=time.perf_counter()for_inrange(iterations):cls(1,2,3)cls(4,5,6)cls(7,8,9)cls(10,11,12)cls(13,14,15)returntime.perf_counter()-startrun_benchmark(10000)#warm it upITERS=1_000_000print(f"Time:{run_benchmark(ITERS):.4f}s")</details>