Allow using active host context in hostfxr_get_runtime_delegate (#86460)

Allow using active host context in hostfxr_get_runtime_delegate (#86460)

These changes allow for passing `nullptr` for the handle parameter of the `hostfxr_get_runtime_delegate` function. This allows in-process callers who do not have the real host context handle to invoke this function and get a runtime delegate for the primary runtime instance. Calling hostfxr_get_runtime_delegate without the handle will only work if there is an active host context, otherwise it will return StatusCode::HostInvalidState.