#28408 Validate sparse tensor external file paths

Description

Adds path traversal validation for sparse tensors with external data, closing a gap where SparseTensorProtoToDenseTensorProto would read external files without checking whether the path escapes the model directory.

Bug fix (pre-existing)

• CopySparseData indices size check: The raw_data().size() check was wrong for external data (where raw_data is empty). Fixed by adding a pre-unpack raw_data size guard for inline data and a post-unpack unpack_buffer size check for all data sources.

Tests

• Security tests (tensorutils_test.cc): Path traversal blocked (values, indices), absolute path blocked (values, indices), zero-element regression (zero dense elements, zero NNZ). All create escaping files and assert specifically for "escapes" error.
• Positive tests (sparse_kernels_test.cc): 7 end-to-end tests for legitimate sparse tensors with external data — external values, external indices (INT64/INT32/INT16/INT8), both external (rank-1 and rank-2 COO).

Known limitation (deferred)

ORT_MEM_ADDR in-memory external data for sparse tensors can trigger arbitrary memory reads. This is a separate issue from path validation — LoadSparseInitializerOrtFormat legitimately uses in-memory markers for ORT-format models, so blanket rejection would break functionality. Should be addressed in a separate PR.

Motivation and Context

A malicious ONNX model could use ../ path traversal in sparse tensor external data locations to read arbitrary files outside the model directory. Dense tensors already had this validation; sparse tensors did not.