fix: align reasoning field name in request & response

[BugenZhao]

Currently it's reasoning_content in the request schema and reasoning in the response schema, so it cannot survive any multi-turn conversation!

[gemini-code-assist]

Code Review

This pull request renames the reasoning_content field to reasoning in the ChatMessage enum and adds a serde alias to maintain backward compatibility. It also updates the conversion logic and unit tests to reflect this change, including a new test case for the legacy alias. The reviewer suggests reverting the primary field name to reasoning_content to maintain alignment with the OpenAI API standard, while using reasoning as the alias instead.