Pass through reasoning input flag and output chunks

[jhrozek]

Some OpenRouter models allow to include the reasoning output if told to do so with a special input flag. This seems to be specific to OpenRouter.

A simple test is:

curl http://localhost:8989/openrouter/completions \
 -H "Authorization: Bearer $OPENROUTER_KEY" \
 -H "Content-Type: application/json" \
 -d '{
   "model": "deepseek/deepseek-r1",
   "messages": [
     {"role": "user", "content": "What is heavier, a kilogram of iron or a kilogram of feathers?"}
   ],
   "include_reasoning": true,
   "stream": true
 }'

Fixes: #977

[jhrozek]

I should note that I didn't find it easy to trigger the reasoning output with Cline lately. I remember seeing it with vanilla openrouter (not through codegate), but I have not during this testing. I see the chunks during curl testing and they seem the same as when directly going to openrouter. Maybe it's just the queries I've been sending to Cline?