Feature Request: Port upstream Anthropic Messages API support

[hksdpc255]

Prerequisites

• I am running the latest code. Mention the version if possible as well.
• I carefully followed the README.md.
• I searched using keywords relevant to my issue to make sure that I am creating a new issue that is not already open (or closed).
• I reviewed the Discussions, and have a new and useful enhancement to share.

Feature Description

Anthropic’s Messages API provides a more unified and consistent way to represent interleaved thinking blocks compared to the OpenAI-style API.

Motivation

Models such as MiniMax-M2 and Kimi-K2-Thinking could benefit from this, since both rely heavily on multi-segment reasoning structures.

Additionaly, this would at least make Claude-Code work with ik_llama.cpp out-of-the-box.

Possible Implementation

See: ggml-org/llama.cpp#17425