[WebGPU] Fix OrtReleaseEnv self-deadlock after failed adapter request on Linux

[Copilot]

On Linux with the WebGPU EP, if SessionOptionsAppendExecutionProvider is called when no Vulkan adapter is available, a subsequent OrtReleaseEnv on the same thread hangs forever in a futex wait.

Root causes

1. C++ exceptions thrown inside Dawn WaitAny callbacks (primary)

ORT_ENFORCE was called directly inside the RequestAdapter/RequestDevice callback lambdas. Dawn's WaitAny does not release its internal EventManager mutex on exception, so throwing through it leaves that mutex permanently locked. The deadlock fires later when Cleanup() calls wgpuInstanceRelease → EventManager::ShutDown() → tries to re-acquire the same mutex on the same thread.

2. Zombie WebGpuContext left in the factory map (secondary)

When Initialize() threw, the WebGpuContext entry remained in contexts_ with ref_count=1 and no owner — a resource leak that would also re-trigger the deadlock path at Cleanup().

Changes (webgpu_context.cc)

• Adapter callback: replace the throwing lambda with a non-throwing one that writes into a local RequestAdapterResult struct; ORT_ENFORCE is moved to after WaitAny returns:

  // Before — throws inside Dawn's callback dispatch:
  [](wgpu::RequestAdapterStatus status, wgpu::Adapter adapter,
     wgpu::StringView message, wgpu::Adapter* ptr) {
      ORT_ENFORCE(status == wgpu::RequestAdapterStatus::Success, ...);
      *ptr = std::move(adapter);
  }, &adapter
  
  // After — captures result, throws outside Dawn:
  [](wgpu::RequestAdapterStatus status, wgpu::Adapter adapter,
     wgpu::StringView message, RequestAdapterResult* result) {
      result->status = status;
      if (status == wgpu::RequestAdapterStatus::Success)
          result->adapter = std::move(adapter);
      else
          result->message = std::string{message};
  }, &adapter_result
  // ORT_ENFORCE(adapter_result.status == ...) called here, after WaitAny

• Device callback: same pattern applied to RequestDevice.

• CreateContext cleanup: wrap Initialize() in a try/catch; on failure decrement ref_count and erase the entry from contexts_ if it reaches zero.

Motivation and Context

Fixes the hang reported when registering the WebGPU EP with no usable Vulkan adapter and then calling OrtReleaseEnv. The process would park on a futex with __owner set to its own TID — a non-recursive mutex locked twice on the same thread — and never exit.