Gate the native engine on the host OS, not the active build target

[MonoFinity]

The problem

UWB picks its native engine off the Unity active build target, but the engine is a native child process of the running Unity process — so in the Editor, the platform that matters is the one the Editor itself is running on.

Because the selection is keyed on UNITY_STANDALONE_* alone, a Windows Editor whose active build target is WebGL, Android or a console compiles:

• WebBrowserUtils.GetRunningPlatform() down to throw new PlatformNotSupportedException(), and
• EngineProcess's constructor down to nothing, leaving processHandle null so every member forwarding to it throws a NullReferenceException.

The practical effect is that UWB cannot be used in the Editor at all by any project that is not currently targeting Standalone. There is no runtime override for it — it's a compile-time gate, so no Engine subclass hook or asmdef arrangement works around it. Switching the build target to Standalone just to preview a webview is disruptive on a large project (it forces an asset re-import and invalidates the platform's script defines).

The change

Widen each gate to UNITY_STANDALONE_X || UNITY_EDITOR_X in the six places that select or compile the per-platform process wrapper:

File	Gate
Helper/WebBrowserUtils.cs	GetRunningPlatform()
Core/Engines/EngineProcess.cs	ctor processHandle selection
Core/Engines/EngineConfiguration.cs	GetEngineExecutableName() — a Windows Editor must still look for the .exe
Core/Engines/Process/WindowProcess.cs	file guard
Core/Engines/Process/LinuxProcess.cs	file guard
Core/Engines/Process/MacOsProcess.cs	file guard

Player builds are unaffected

This only widens what compiles inside the Editor. A Standalone player defines UNITY_STANDALONE_X but not UNITY_EDITOR_X; a WebGL/Android player defines neither. Both keep the existing behaviour byte-for-byte.

Notes

• Verified against master (f49743d).
• We've been running this patch in a vendored copy of 2.2.8 for production Editor work on Windows, hosting a CEF-rendered React UI in the Game View with the build target set to WebGL.
• Sent as a separate PR from Bound the TCP client's connect timeout instead of int.MaxValue #440 — the two are independent and either can be taken without the other.