Limit nested model graph depth (#32344)
This pull request introduces a validation step to limit the maximum
allowed subgraph nesting depth in ONNX models to 32. This prevents
excessively deep or recursive subgraph structures that could cause stack
overflows or performance issues. The change is enforced during model
loading and is covered by new unit tests.
**Model validation enhancements:**
* Added a new function `ValidateModelSubgraphDepth` that checks the
nesting depth of subgraphs in a model and returns an error if the depth
exceeds `kMaxModelSubgraphDepth` (32).
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* Integrated `ValidateModelSubgraphDepth` into the `Model` constructor
to enforce subgraph depth checks during model loading.
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**Testing improvements:**
* Added unit tests to verify that models with excessive subgraph depth
are correctly rejected, and that the depth limit is enforced for both
regular subgraphs and those in local functions.
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* Included necessary header for `model_helpers.h` in test files to
support new validation logic.
**API and constant definition:**
* Introduced the constant `kMaxModelSubgraphDepth` in `model_helpers.h`
to define the supported subgraph depth limit.
* Declared the new validation function in the header for use across the
codebase.
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