Use single extract questions method across all LLMs for doc search

Using model specific extract questions was an artifact from older
times, with less guidable models.

New changes collate and reuse logic
- Rely on send_message_to_model_wrapper for model specific formatting.
- Use same prompt, context for all LLMs as can handle prompt variation.
- Use response schema enforcer to ensure response consistency across models.

Extract questions (because of its age) was the only tool directly within
each provider code. Put it into helpers to have all the (mini) tools
in one place.

tests/test_online_chat_actors.py

@@ -4,10 +4,11 @@ import freezegun
 import pytest
 from freezegun import freeze_time
 
-from khoj.processor.conversation.openai.gpt import converse_openai, extract_questions
+from khoj.processor.conversation.openai.gpt import converse_openai
 from khoj.processor.conversation.utils import message_to_log
 from khoj.routers.helpers import (
     aget_data_sources_and_output_format,
+    extract_questions,
     generate_online_subqueries,
     infer_webpage_urls,
     schedule_query,