[VitisAI] Recognize native-endian in-memory address tag in process_ext_address

[mingyueliuh]

Description

vaip::process_ext_address (in the VitisAI EP) decodes the in-memory
"external data" address marker that ORT plants on initializers whose data lives
in an in-process buffer. It only matched the little-endian tag
"*/_ORT_MEM_ADDR_/*" (kTensorProtoLittleEndianMemoryAddressTag). This PR
makes it also recognize the native-endian tag
"*/_ORT_NATIVE_ENDIAN_MEM_ADDR_/*" (kTensorProtoNativeEndianMemoryAddressTag).

if (file == "*/_ORT_MEM_ADDR_/*" || file == "*/_ORT_NATIVE_ENDIAN_MEM_ADDR_/*") {
  auto addr = reinterpret_cast<const char*>(offset);
  return {addr, size};
}

Motivation and Context

Two existing changes combined to break VitisAI model compilation:

1. S390x test fixes #27404 split the single in-memory marker into a little-endian and a
   native-endian variant, and switched TensorToTensorProto(use_tensor_buffer=true)
   to emit the native-endian tag by default. Both HasExternalDataInMemory
   and the data readers treat the two tags equivalently.

2. Address external path references in loaded models #28709 added an explicit ORT_ENFORCE(!utils::HasExternalDataInMemory(tensor), ...)
   in Graph::Graph, rejecting any in-memory address marker found in a
   deserialized model protobuf.

Because process_ext_address still only recognized the little-endian tag,
in-memory initializers (e.g. quantized weights moved out of the TensorProto
during graph optimization) are no longer detected in vaip::model_clone. They
fall through to the verbatim-copy branch and the native-endian marker is carried
into the cloned model proto. When ORT then constructs the cloned Graph, the
new enforce fires and aborts compilation:

Initializer 'onnx::Conv_1575_quantized' references an ORT in-memory address
marker, which is not allowed in a model protobuf.

This regression is observed on 1.27.0, where both changes are present (1.25.x /
1.26.x have the native-endian tag but not the enforce, so the marker leaked
silently without crashing).

Both tags encode the buffer address in the offset field; on little-endian
hosts (the only platform this EP targets) the byte layout is identical, so they
are decoded the same way. Recognizing both tags restores the previous behavior
of converting these initializers into a lightweight reference inside
model_clone, so the in-memory marker never reaches Graph::Graph.

[mingyueliuh]

@adrianlizarraga @yuslepukhin please take a look when you are available. Thanks.