feat(service): add ForwardAsResponses/ForwardAsChatCompletions on GatewayService

New forwarding methods on GatewayService for Anthropic platform groups:

- ForwardAsResponses: accept Responses body → convert to Anthropic →
  forward to upstream → convert response back to Responses format.
  Supports both streaming (SSE event-by-event conversion) and buffered
  (accumulate then convert) response modes.
- ForwardAsChatCompletions: chain CC→Responses→Anthropic for request,
  Anthropic→Responses→CC for response. Streaming uses dual state machine
  chain with [DONE] marker.

Both methods reuse existing GatewayService infrastructure:
buildUpstreamRequest, Claude Code mimicry, cache control enforcement,
model mapping, and return UpstreamFailoverError for handler-level retry.

Author: Ethan0x0000