[GR-76116] Remove Truffle NFI dependency and references.

[graalvmbot]

Cleanup NFI usage after previous migrations.
Removing Truffle NFI dependency reduces native image size by about 5% on my box.

Change summary:

• Removes the Truffle NFI dependency from GraalPy:
• Removes the NativePointer wrapper and replaces related context/native-resource handling with primitive long.
• Refactors C API transition code to remove interop pointer cases
• Uses PythonToNativeInternalNode and NativeToPythonInternalNode wherever possible
• Introduces convenience methods PythonToNativeInternalNode.executeNewRef and NativeToPythonInternalNode.executeTransfer to improve readability
• Cleans up obsolete executeRaw / executeLong methods
• Cleans up CExtToJavaNode / CExtToNativeNode usage and hierarchy. Use primitive long for pointers in the execute method signatures.
• Refactors conversion logic in ReadMemberNode and avoids unnecessary conversion nodes for primitives.