Fix OpenVINO provider info namespace for Python builds (#33327)
## Description
Fixes #33325.
`ProviderInfo_OpenVINO` is a global interface declared by the OpenVINO
provider factory header. #33112 added a forward declaration inside
`namespace onnxruntime`, creating a different type. That conflicts with
the existing Python binding declaration of `TryGetProviderInfo_OpenVINO`
and causes MSVC C2556 when building with OpenVINO and Python enabled.
Move the interface forward declaration to global scope while retaining
the function in `namespace onnxruntime`. Make the existing
optional-provider probe test use an explicitly global interface pointer
so a namespace mismatch fails compilation.
## Validation
- Windows Visual Studio 18 2026: built the optional-provider probe
targets in normal and no-exceptions modes.
- Both corresponding CTest suites passed.
- clang-cl declaration probes passed in both
provider-bridge/Python-binding include orders.
- Scoped lintrunner and `git diff --check` passed.
The full OpenVINO Python wheel build was not run; no OpenVINO SDK was
installed for this validation.