feat: add Anthropic Messages to OpenAI ChatCompletions translator

[khshanovskyi]

Applicable issues

• fixes #

Description of changes

Add a FastAPI sub-app (aidial_adapter_bedrock/anthropic_translator/chat_completions/)
mounted at /to-chat-completions/anthropic, translating Anthropic Messages
requests (/v1/messages) into OpenAI Chat Completions API calls against
DIAL Core ({DIAL_URL}/openai/deployments/{deployment}/chat/completions)
and translating the streaming/non-streaming responses back into Anthropic
Messages shape. This lets an Anthropic-native client (e.g. Claude Code)
talk to a model that only speaks the OpenAI Chat Completions API.

• Support the bulk of the Anthropic Messages surface: system prompt
  (string/blocks, including mid-conversation system-role turns merged
  into one leading system message), user/assistant messages, tool_result/
  tool_use, image/document blocks, tool_choice, reasoning-effort mapping
  from output_config.effort, and cache_control propagated as DIAL Core's
  custom_fields.cache_breakpoint. Unsupported features (top_k, thinking
  budget, web_search/bash/text_editor/computer/code_execution tools,
  mcp_servers, container) are dropped with a warning instead of being
  rejected, so clients keep working. There is no count_tokens endpoint,
  since Chat Completions has no token-counting API.

• Map DIAL Core and Chat Completions errors into Anthropic-shaped error
  responses, for both the HTTP error body and the terminal SSE error
  event.

• Promote openai from a test-only to a runtime dependency (1.107.3 ->
  2.40.0) to reuse its typed Chat Completions client and SSE handling
  for the outbound call to Core.

• Document the new interfaces.anthropicMessages DIAL Core config
  pattern and the translator's feature-support/limitations table in
  README.md.

• Add unit tests covering request/response conversion, the streaming
  state machine, error mapping, and the mounted endpoints.

Checklist

• Title of the pull request follows Conventional Commits specification

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