feat(apicompat): add Responses↔Anthropic bidirectional format conversion

Add reverse-direction converters for Anthropic platform groups to accept
OpenAI-format requests:

- ResponsesToAnthropicRequest: Responses API input → Anthropic Messages
  request with system extraction, tool/toolChoice mapping, reasoning
  effort conversion, image data URI↔base64, and consecutive role merging
- AnthropicToResponsesResponse: Anthropic response → Responses response
  with content block→output item mapping, usage, stop_reason→status
- AnthropicEventToResponsesEvents: stateful SSE stream converter
  (Anthropic streaming protocol → Responses streaming protocol)
- FinalizeAnthropicResponsesStream: synthetic termination for
  incomplete streams

Author: Ethan0x0000