[Metal] Make dispatchRays a self-contained encoder operation

[MarijnS95]

The DX and Vulkan backends record a ray dispatch by creating an encoder and calling Encoder.dispatchRays() directly. Metal instead routed through createRayTracingCommands, an MTLDevice method that bound the descriptor heap and top-level argument buffer, synthesized and bound the IRDispatchRaysArgument, marked the RT resources resident, and only then called a thin dispatchRays().

Since Metal lowers raygen to a compute kernel, ray tracing now records through the shared createComputeCommands path — which already binds the heap and argument buffer and marks the common resources resident — by branching on P.isRayTracing(). The IRDispatchRaysArgument construction, upload, binding, and RT-specific residency move into MTLComputeEncoder::dispatchRays, defined out-of-line so it can allocate via CB->Dev and keep the buffer alive in CB->KeepAliveOwned. IRDispatchRaysArgument::ResDescHeap needs the resource heap address, which isn't in the cross-backend dispatchRays signature, so the encoder remembers the bound heap via bindResourceHeap(). createRayTracingCommands is removed.

No behavioral change. check-hlsl-mtl-feature-rt and check-hlsl-mtl-basic pass.