Pin C# RunAsync arguments until completion (#32015)
This pull request improves the memory management and safety of async
inference execution in the ONNX Runtime C# API, particularly around the
`RunAsync` method. The main changes ensure that arrays passed to native
code are properly pinned and unpinned, reducing the risk of memory
errors, and that resources are disposed of correctly in both success and
failure scenarios. The test code is also updated to more rigorously
exercise object lifetimes.
### Memory management and resource safety improvements
* The `CallbackHost` class now implements `IDisposable` and manages the
pinning and unpinning of input and output arrays using `GCHandle`,
ensuring that native code receives stable pointers and that handles are
freed even on exceptions.
[[1]](diffhunk://#diff-622c8600020a433468d4a0e82a320fdc478e0c685c2d92c86c04dac351af4d04L1146-R1150)
[[2]](diffhunk://#diff-622c8600020a433468d4a0e82a320fdc478e0c685c2d92c86c04dac351af4d04R1162-R1176)
[[3]](diffhunk://#diff-622c8600020a433468d4a0e82a320fdc478e0c685c2d92c86c04dac351af4d04R1188-R1268)
* The `OrtCallback` and `RunAsyncInternal` methods are updated to
explicitly dispose of the `CallbackHost` and free handles in all code
paths, further strengthening resource safety.
[[1]](diffhunk://#diff-622c8600020a433468d4a0e82a320fdc478e0c685c2d92c86c04dac351af4d04R1135)
[[2]](diffhunk://#diff-622c8600020a433468d4a0e82a320fdc478e0c685c2d92c86c04dac351af4d04R1188-R1268)
### Native interop changes
* The native P/Invoke signature for `OrtRunAsync` is updated to pass
pointers (`IntPtr`) instead of managed arrays, matching the new pinned
memory approach.
* Calls to the native API (`OrtRunAsync`) are updated to use the new
pointer properties from `CallbackHost` instead of directly passing
arrays.
### Test improvements
* The async inference test now explicitly creates and nulls out a
`RunOptions` instance and forces garbage collection, testing that
managed resources are not prematurely collected while native work is
outstanding.
These changes collectively make async inference safer and more robust,
especially in scenarios with concurrent or long-running operations.
