[Bug]: ArrayIndexOutOfBoundsException with `__build_class__`

Start by running the provided build_class reproducer on GraalPy and trace the failure from PArguments.getArgument through PBytecodeDSLRootNode$CopyArguments. Investigate the argument handling involved in this call and add a regression check if the relevant test location is identified. Done means the snippet completes without an ArrayIndexOutOfBoundsException.

Describe the bug

GraalPy crashes when running this fuzzed code:

def fn(x):
    pass

__build_class__(fn, '', '')

Operating system

Linux

CPU architecture

x86_64

GraalPy version

GraalPy 3.13.14 (Oracle GraalVM Native 25.3.4.1)

JDK version

No response

Context configuration

No response

Steps to reproduce

Snippet listed above

Expected behavior

No crash

Stack trace

java.lang.ArrayIndexOutOfBoundsException: Stack trace is imprecise, the top frames are missing and/or have wrong line numbers. To get precise stack traces, build the image with option -H:-ReduceImplicitExceptionStackTraceInformation
	at org.graalvm.py/com.oracle.graal.python.builtins.objects.function.PArguments.getArgument(PArguments.java:246)
	at org.graalvm.py/com.oracle.graal.python.builtins.objects.function.PArguments.getArgument(PArguments.java:250)
	at org.graalvm.py/com.oracle.graal.python.nodes.bytecode_dsl.PBytecodeDSLRootNode$CopyArguments.perform(PBytecodeDSLRootNode.java:1599)
	at org.graalvm.py/com.oracle.graal.python.nodes.bytecode_dsl.PBytecodeDSLRootNodeGen$CopyArguments_Node.execute$uncached(PBytecodeDSLRootNodeGen.java)
	at org.graalvm.py/com.oracle.graal.python.nodes.bytecode_dsl.PBytecodeDSLRootNodeGen$UncachedBytecodeNodeTailCall.handleCopyArguments_(PBytecodeDSLRootNodeGen.java:36250)
	at org.graalvm.py/com.oracle.graal.python.nodes.bytecode_dsl.PBytecodeDSLRootNodeGen$UncachedBytecodeNodeTailCall.__stub_handleCopyArguments_(PBytecodeDSLRootNodeGen.java:0)
	at org.graalvm.py/com.oracle.graal.python.nodes.bytecode_dsl.PBytecodeDSLRootNodeGen$UncachedBytecodeNodeTailCall.continueAt(PBytecodeDSLRootNodeGen.java:35093)
	at org.graalvm.py/com.oracle.graal.python.nodes.bytecode_dsl.PBytecodeDSLRootNodeGen.continueAt(PBytecodeDSLRootNodeGen.java:3501)
	at org.graalvm.py/com.oracle.graal.python.nodes.bytecode_dsl.PBytecodeDSLRootNodeGen.execute(PBytecodeDSLRootNodeGen.java:3493)
	at org.graalvm.truffle.runtime/com.oracle.truffle.runtime.OptimizedCallTarget.executeRootNode(OptimizedCallTarget.java:808)
	at org.graalvm.truffle.runtime/com.oracle.truffle.runtime.OptimizedCallTarget.profiledPERoot(OptimizedCallTarget.java:722)
	at org.graalvm.truffle.runtime/com.oracle.truffle.runtime.OptimizedCallTarget.callBoundary(OptimizedCallTarget.java:641)
	at org.graalvm.truffle.runtime.svm/com.oracle.svm.truffle.api.SubstrateOptimizedCallTarget.invokeCallBoundary(SubstrateOptimizedCallTarget.java:124)
	at com.oracle.truffle.enterprise.svm/com.oracle.svm.enterprise.truffle.compiler.SubstrateEnterpriseOptimizedCallTarget.invokeCompiledAdapterOrCallBoundary(SubstrateEnterpriseOptimizedCallTarget.java:293)
	at com.oracle.truffle.enterprise.svm/com.oracle.svm.enterprise.truffle.compiler.SubstrateEnterpriseOptimizedCallTarget.invokeFromInterpreter(SubstrateEnterpriseOptimizedCallTarget.java:269)
	at com.oracle.truffle.enterprise.svm/com.oracle.svm.enterprise.truffle.compiler.SubstrateEnterpriseOptimizedCallTarget.doInvoke(SubstrateEnterpriseOptimizedCallTarget.java:255)
	at org.graalvm.truffle.runtime/com.oracle.truffle.runtime.OptimizedCallTarget.callDirect(OptimizedCallTarget.java:573)
	at org.graalvm.truffle.runtime/com.oracle.truffle.runtime.OptimizedIndirectCallNode.call(OptimizedIndirectCallNode.java:68)
[...snip...]

Additional context

No response