Fix DllImportResolver (#27397)
### Description
This PR addresses two issues related to the newly added
`DllImportResolver` for `.NET` native library loading:
1. **Fix IL3000 Warning during Native AOT / Single-File Publish**
When publishing projects that reference `Microsoft.ML.OnnxRuntime` as a
single file or using Native AOT in .NET 9, the compiler reports an
`IL3000` warning/error because `DllImportResolver` accesses
`Assembly.Location`. In these deployment models, `Assembly.Location`
always returns an empty string or throws.
Since `DllImportResolver` already correctly handles the empty string
failure and falls back to `AppContext.BaseDirectory` (which is fully
supported), this PR adds the `[UnconditionalSuppressMessage]` attribute
to suppress the build warning statically.
2. **Fix `TypeInitializationException` in `NativeMethods` Static
Constructor**
Users reported a `System.TypeInitializationException: The type
initializer for 'Microsoft.ML.OnnxRuntime.NativeMethods' threw an
exception.` when initializing the ONNX Runtime environment.
This occurs because the `DllImportResolver` (registered in the static
constructor) is invoked on the first P/Invoke (`OrtGetApiBase`). If any
API within the resolver throws an unhandled exception (for instance,
`AppContext.BaseDirectory` throwing `AppDomainUnloadedException` in
sandboxed AppDomains or `Environment.GetEnvironmentVariable` throwing
`SecurityException`), the exception bubbles up and crashes the
application with a type initialization failure.
This PR wraps the `DllImportResolver` logic in a `try-catch` block
(specifically handling `AppContext.BaseDirectory` edge cases) so that
any resolution failure safely swallows the error and falls back to
`IntPtr.Zero`, allowing the default .NET Platform Invoke mechanism to
take over and throw a standard `DllNotFoundException` instead of a fatal
type initialization crash.
A unit test (`TestDllImportResolverDoesNotThrow`) has been added to
`OrtEnvTests.cs` to verify that `DllImportResolver` successfully
swallows internal exceptions without crashing the initialization
process.
### Motivation and Context
These changes ensure that .NET developers can safely compile Native
AOT/Single-File applications without build errors and prevent hard
application crashes in environments with restricted permissions.
