Implement compiled model compatibility APIs in example plugin EP and add tests (#27088)
### Description
<!-- Describe your changes. -->
Add tests for the compiled model compatibility APIs
(`GetCompiledModelCompatibilityInfo` and
`ValidateCompiledModelCompatibilityInfo`) using the example plugin EP.
Changes:
- Implement the compatibility methods in example_plugin_ep to enable
testing
- Update `GetCapabilityImpl` and `CompileImpl` to handle EPContext nodes
(required for loading compiled models in tests)
- Add `PluginEp_CompatibilityInfo_WrittenToMetadata` test - verifies
compatibility info is written to compiled model metadata
- Add `PluginEp_CompatibilityInfo_ValidatedOnLoad` test - verifies the
round-trip: compile → load → validate compatibility
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
The compiled model compatibility APIs need test coverage to ensure they
work correctly end-to-end. This exercises the full flow of writing
compatibility metadata during compilation and validating it when loading
a pre-compiled model. Additionally, by adding this support into the
plugin EP, it enables us to add scenario tests that would exercise this
in the context of Windows ML.
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Co-authored-by: Aditya Rastogi <adityar@ntdev.microsoft.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>