Review and document ONNX testing coverage

[matouskozak]

ONNX tests exist across transformer, integration, conversion, and sequence-type test projects. However, there is no current coverage matrix for supported scenarios, validation paths, platforms, and public APIs.

Issues such as #2500 and #3375 also show areas that need review.

Goal

Review the current ONNX test suite, document its coverage, and create focused follow-up issues for confirmed gaps.

Scope

Review coverage for:

• Model loading from files, streams, and memory
• Fixed, dynamic, unknown, and variable-length dimensions
• Tensor, sequence, and map types
• Single and multiple inputs and outputs
• Input and output schema validation
• Image layout, channel order, and normalization
• Model save, load, disposal, and session options
• CPU and GPU execution and fallback
• Windows, Linux, and macOS
• Output correctness against expected ONNX Runtime results
• Use of current public APIs

Acceptance criteria

• Create a matrix that maps each scenario to its tests and platforms.
• Review OnnxTransform - Ubuntu-only unit tests and friendly error message on incompatible Linux distributions #2500 and Add OnnxTransform test for variable length vectors #3375 and record their current status.
• Identify obsolete, duplicate, skipped, or platform-limited tests.
• Identify tests that do not validate output correctness.
• Create a focused follow-up issue for each confirmed gap, or document why no test is necessary.

[github-actions]

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