Providers like Moonshot (Kimi K2.5/K2.6) require the reasoning_content
field to be present on assistant tool-call messages in multi-turn
conversations. The sanitizer's allow-list was missing this field,
causing HTTP 400: 'thinking is enabled but reasoning_content is missing
in assistant tool call message at index N'.
Add reasoning_content to the allowed field set in
_sanitize_llm_messages and cover with regression tests.
Fixes#3118
Co-authored-by: michaelxer <michaelxer@users.noreply.github.com>
Co-authored-by: Alexandre Teixeira <111787685+alteixeira20@users.noreply.github.com>
Michael