Validate NodeArg creation (#28429)
This pull request improves the robustness of ONNX model loading and
validation by adding stricter checks for malformed models and enhancing
test coverage. The most important changes are:
**Model Loading Robustness:**
* Added an `ORT_ENFORCE` check in
`Graph::InitializeStateFromModelFileGraphProto()` to ensure that every
initializer in the model has a corresponding `NodeArg`, preventing
crashes when loading malformed models.
**Test Coverage for Malformed Models:**
* Added a test (`MalformedModelInitializerWithoutNodeArg`) to verify
that a model with an initializer lacking a matching `NodeArg` is
properly rejected during loading, ensuring graceful error handling.
* Added a test (`MalformedModelEmptySubgraph`) to verify that a model
with a node containing an empty subgraph attribute is rejected, further
ensuring structural validation of models.
