fix(claude): accumulate consecutive role turns during request conversion

- add `ClaudeMessageAccumulator` to merge adjacent Claude `user`/`assistant` messages and preserve role boundaries
- preserve `assistant` content order by moving `tool_use` parts to the end of each assistant turn
- switch OpenAI chat-completions, Interactions, and Gemini Claude request translators to use the shared accumulator for consistent turn grouping/flush behavior