[Plugin EP] Session options getters (#28377)
## Description
Add getter APIs for `MemPatternEnabled` and `ExecutionMode` session
options to allow Execution Providers to query these settings.
### Motivation
Plugin EPs need to inspect session option values in order to validate
compatibility. For example, EPs whose memory is not byte-addressable
(like DML and WebGPU) are incompatible with the memory pattern
optimization, and some EPs do not support parallel execution mode. For
example:
https://github.com/microsoft/onnxruntime/blob/28bcc9cf304b3af4eece60700c09f2e2561ba655/onnxruntime/core/session/inference_session.cc#L901-L920
Currently, the C API only exposes setters
(`EnableMemPattern`/`DisableMemPattern`, `SetSessionExecutionMode`) with
no corresponding getters. This means plugin EPs loaded from shared
libraries have no way to query these values and return an appropriate
error status if the session options are incompatible.
### Changes
Added two new C API functions (Version 1.27):
- **`GetMemPatternEnabled`** — Returns whether the memory pattern
optimization is enabled (1) or disabled (0).
- **`GetSessionExecutionMode`** — Returns the current execution mode
(`ORT_SEQUENTIAL` or `ORT_PARALLEL`).
Both APIs also have corresponding C++ wrappers:
- `bool SessionOptions::GetMemPatternEnabled() const`
- `ExecutionMode SessionOptions::GetExecutionMode() const`
### Files Changed
| File | Description |
|------|-------------|
| `include/onnxruntime/core/session/onnxruntime_c_api.h` | New C API
declarations with documentation |
| `onnxruntime/core/session/ort_apis.h` | Internal API declarations |
| `onnxruntime/core/session/abi_session_options.cc` | Implementations |
| `onnxruntime/core/session/onnxruntime_c_api.cc` | API table
registration |
| `include/onnxruntime/core/session/onnxruntime_cxx_api.h` | C++ wrapper
declarations |
| `include/onnxruntime/core/session/onnxruntime_cxx_inline.h` | C++
inline implementations |
| `onnxruntime/test/shared_lib/test_session_options.cc` | Unit tests |
### Testing
Added unit tests that verify:
- `GetMemPatternEnabled`: default is enabled, disable returns false,
re-enable returns true
- `GetSessionExecutionMode`: default is `ORT_SEQUENTIAL`, set to
`ORT_PARALLEL` and back
