feat: add reasoning token support to OpenRouter integration

Extract reasoning/thinking content from OpenRouter responses and attach
to ChatMessage via ReasoningContent. Override run(), run_async(), and
stream handler. Handle multi-turn by re-injecting reasoning_details
into formatted message dicts.

13 new tests covering extraction, conversion, streaming, multi-turn,
edge cases, and async paths.

Closes #2181

Author: ArkaD171717