Add model-ingestion error-path tests for InferenceSession

[GopalakrishnanN]

Description

Extends error-path / negative test coverage for InferenceSession model ingestion (continuation of the audit's "thin error-path coverage" finding). Adds two tests to inference_session_test.cc:

• LoadModelTwiceReturnsError — a second Load() on a session that already holds a model must be rejected with MODEL_LOADED ("This session already contains a loaded model").
• LoadInvalidGraphReturnsError — a model whose only node consumes an input that is never defined (not a graph input, initializer, or another node's output) is constructed via the ONNX proto API, serialized, and loaded. Graph Resolve must reject it gracefully (error at Load or Initialize) rather than crash or silently accept it.

Motivation and Context

Failure/validation paths are under-tested relative to the numeric happy-path bulk. These cover the double-load guard and the graph-resolve rejection path for a malformed graph.

Verification

Built onnxruntime_test_all (Release, CPU) and ran both tests locally — both pass:

[ OK ] InferenceSessionTests.LoadModelTwiceReturnsError
[ OK ] InferenceSessionTests.LoadInvalidGraphReturnsError
[ PASSED ] 2 tests.

[Copilot]

Pull request overview

This PR extends ONNX Runtime’s InferenceSession negative/error-path test coverage around model ingestion, ensuring common misuse/malformed-model scenarios are rejected cleanly.

Changes:

• Adds a test that a second Load() call on the same InferenceSession is rejected.
• Adds a test that loading/initializing a malformed ONNX model (node consumes an undefined input) fails gracefully.