[C#] Add EP tests for Cuda Plugin (#28375)
This pull request adds comprehensive test coverage and documentation for
the CUDA Plugin Execution Provider (EP) in the C# ONNX Runtime codebase.
It introduces new tests to validate registration, session creation,
inference, device properties, provider options, automatic EP selection,
and I/O binding with the CUDA Plugin EP. Additionally, it updates the
documentation to clarify the correct registration name and adds a new
test dependency.
**CUDA Plugin Execution Provider support and testing:**
* Added a new test file `CudaPluginEpTests.cs` with multiple
[SkippableFact] tests covering registration, inference, device
properties, provider options, automatic EP selection, and I/O binding
for the CUDA Plugin Execution Provider. These tests are skipped if the
required library is unavailable.
* Updated the test project file
`Microsoft.ML.OnnxRuntime.Tests.Common.csproj` to include the
`Xunit.SkippableFact` package, enabling conditional test skipping.
**Documentation:**
* Added a section to `CONTRIBUTING.md` specifying the correct
registration name for the CUDA Plugin Execution Provider as
`CudaPluginExecutionProvider`.
**Minor code cleanup:**
* Removed unused imports from `OrtEnvTests.cs` for improved code
clarity.
