UnregisterExecutionProviderLibrary can access torn-down CRT state during Windows process shutdown

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### Describe the issue

Calling `OrtApi::UnregisterExecutionProviderLibrary` from late process-shutdown cleanup can crash inside ONNX Runtime.

ONNX Runtime already detects Windows process termination in `onnxruntime/core/dll/dllmain.cc`: when `DLL_PROCESS_DETACH` has a non-null reserved parameter, it sets `g_is_shutting_down`. `OrtEnv::Release` uses this state to intentionally skip destruction during process termination

However, `OrtApi::UnregisterExecutionProviderLibrary` does not check that state before accessing `OrtEnv::GetEnvironment()`. A consumer that explicitly unregisters an EP from global or DLL teardown can therefore enter `Environment::UnregisterExecutionProviderLibrary` after CRT state has begun shutting down.

This occurs across all EPs

The original failure is:

- Exception: `0xC0000005` access violation
- Faulting code: `onnxruntime!std::ctype::widen`
- Invalid read address: `0xFFFFFFFFFFFFFFFF`
- AppVerifier state included `0xF0F0F0F0F0F0F0F0`, consistent with invalid or poisoned teardown state
- Call path:
`onnxruntime::Environment::UnregisterExecutionProviderLibrary`
-> consumer execution-provider cleanup
-> global destructor
-> `LdrShutdownProcess`

I have included a potential fix PR here: https://github.com/microsoft/onnxruntime/pull/32401

### To reproduce

1. Build or use ONNX Runtime as a Windows shared library.
2. Create an `OrtEnv`.
3. Dynamically register an execution-provider library using `RegisterExecutionProviderLibrary`.
4. Retain the environment and registration name in a consumer DLL or global owner.
5. From that owner's destructor, call `UnregisterExecutionProviderLibrary`.
6. Arrange for ONNX Runtime to receive `DLL_PROCESS_DETACH` for process termination before the consumer's global destructor runs. This can occur when ONNX Runtime is delay-loaded by the consumer.
7. Exit the process with Application Verifier enabled.

### Urgency

_No response_

### Platform

Windows

### OS Version

26683.1002

### ONNX Runtime Installation

Built from Source

### ONNX Runtime Version or Commit ID

1.28.1

### ONNX Runtime API

C++

### Architecture

X64

### Execution Provider

Other / Unknown

### Execution Provider Library Version

_No response_

Read onnxruntime/core/dll/dllmain.cc for g_is_shutting_down, then trace OrtEnv::Release and OrtApi::UnregisterExecutionProviderLibrary into Environment::UnregisterExecutionProviderLibrary. Reproduce the described Windows shared-library teardown with Application Verifier and verify that late unregister cleanup no longer accesses torn-down CRT state.

Written by the indexing model from the issue text.