#28709 Address external path references in loaded models

This pull request strengthens security checks around loading external tensor data in ONNX Runtime, particularly to prevent malicious models from referencing unsafe file paths or in-memory address markers that could lead to arbitrary file access or unsafe memory dereferencing. The changes introduce stricter validation for external data paths and add explicit rejections for ORT in-memory address markers found in model protobufs, along with new and improved regression tests to verify this behavior.

Security hardening for external data loading:

• Added ValidateExternalFilePathForTensor to enforce that external data paths are validated for all code paths loading external data (including those outside Graph::Resolve), rejecting absolute or directory-escaping paths and passing through only trusted in-memory markers. This is now called in GetExtDataFromTensorProto and LoadExtDataToTensorFromTensorProto to ensure defense-in-depth. [1] [2]
• Updated the validation logic for sparse tensor sub-tensors with ValidateSparseSubTensorExternalDataPath, clarifying the handling of in-memory markers and ensuring only legitimate file paths are accepted.
• Changed SparseTensorProtoToDenseTensorProto to use the new sparse sub-tensor validation for both values and indices.

Model loading and graph construction protections:

• In Graph::Graph, added explicit rejection of ORT in-memory address markers in sparse tensor attributes and initializers when loading from a protobuf, preventing attackers from crafting models that could cause unsafe memory access during sparse-to-dense conversion or initializer resolution. [1] [2] [3]

Expanded and improved testing:

• Added new unit tests to verify that absolute and directory-escaping external paths are rejected even when loading tensors directly (not via graph resolution), and that in-memory address markers are not accepted in dense or sparse initializers loaded from protobufs. [1] [2]
• Updated an optimizer initializer test to reflect the new error handling for invalid external data paths.