Enable GraphIterator interface by default

[mvafin]

Details:

• Enable GraphIterator interface by default
• Add tests for legacy behavior in the GHA
• Fix issue in OpExtension test
• Fix issue with Unicode paths

Tickets:

• CVS-154613

[Copilot]

Pull request overview

This PR enables the GraphIterator interface by default for ONNX frontend processing, adds comprehensive testing for legacy behavior in GitHub Actions, and fixes an issue in OpExtension attribute handling.

• GraphIterator is now enabled by default (when ONNX_ITERATOR is not explicitly set to disable it)
• Test infrastructure updated to run both with GraphIterator enabled and disabled
• Python and C++ test helpers added to conditionally skip ONNX Editor tests when GraphIterator is active

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/frontends/onnx/tests/tests_python/test_frontend_onnx_editor.py	Adds helper functions and xfail marks for editor tests that are incompatible with GraphIterator
src/frontends/onnx/tests/tests_python/test_frontend_onnx.py	Adds GraphIterator detection and xfail marker for decode/convert tests
src/frontends/onnx/tests/onnx_utils.hpp	Implements C++ helper function to check if GraphIterator is enabled via environment variable
src/frontends/onnx/tests/onnx_import_org_openvino.in.cpp	Replaces direct std::getenv calls with the new is_graph_iterator_enabled() helper
src/frontends/onnx/tests/onnx_editor_topological_sort.cpp	Adds skip macros to editor tests that are incompatible with GraphIterator
src/frontends/onnx/tests/onnx_editor_test_utils.hpp	Defines macro and skip message for GraphIterator-incompatible editor tests
src/frontends/onnx/tests/onnx_editor.cpp	Adds skip macros to numerous editor tests that don't work with GraphIterator
src/frontends/onnx/frontend/src/frontend.cpp	Changes default behavior to enable GraphIterator and refactors environment variable checking
src/frontends/onnx/frontend/src/core/node.cpp	Fixes attribute value retrieval for decoder-based nodes with float-to-double upcast
.github/workflows/job_python_unit_tests.yml	Duplicates ONNX test runs to cover both GraphIterator enabled and disabled modes
.github/workflows/job_cxx_unit_tests.yml	Duplicates ONNX frontend tests to cover both GraphIterator enabled and disabled modes

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[bumbosiepsak]

I have few questions in few places. Let's clarify and approve afterwads.

[mvafin]

Add a message about deprecation of old approach in future

[bumbosiepsak]

I guess it might actually be better for the user if we exit here (instead of warning and continuing).

Rationale: easy to miss a warning - especially if you don't know what to look for.