Thinking/reasoning models (Kimi K2.5, GLM-4.7, DeepSeek-R1) return a
reasoning_content field in assistant messages containing tool calls.
ZeroClaw was silently dropping this field when constructing conversation
history, causing provider APIs to reject follow-up requests with 400
errors: "thinking is enabled but reasoning_content is missing in
assistant tool call message".
Add reasoning_content: Option<String> as an opaque pass-through at every
layer of the pipeline: ChatResponse, ConversationMessage, NativeMessage
structs, parse/convert/build functions, and dispatcher. The field is
skip_serializing_if = None so it is invisible for non-thinking models.
Closes#1327
7e6491142e