Integrate LiteLLM for multi-provider LLM support (#168)

* Integrate litellm for multi-provider LLM support

* recover the default config yaml

* Use litellm.acompletion for native async support

* fix tob

* Rename llm_complete/allm_complete to llm_completion/llm_acompletion, remove unused llm_complete_stream

* Pin litellm to version 1.82.0

* resolve comments

* args from cli is used to overrides config.yaml

* Fix get_page_tokens hardcoded model default

Pass opt.model to get_page_tokens so tokenization respects the
configured model instead of always using gpt-4o-2024-11-20.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Remove explicit openai dependency from requirements.txt

openai is no longer directly imported; it comes in as a transitive
dependency of litellm. Pinning it explicitly risks version conflicts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Restore openai==1.101.0 pin in requirements.txt

litellm==1.82.0 and openai-agents have conflicting openai version
requirements, but openai==1.101.0 works at runtime for both.
The pin is necessary to prevent litellm from pulling in openai>=2.x
which would break openai-agents.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Remove explicit openai dependency from requirements.txt

openai is not directly used; it comes in as a transitive dependency
of litellm. No openai-agents in this branch so no pin needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix an litellm error log

* resolve comments

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

pageindex/config.yaml

@@ -1,4 +1,5 @@
 model: "gpt-4o-2024-11-20"
+# model: "anthropic/claude-sonnet-4-6"
 toc_check_page_num: 20
 max_page_num_each_node: 10
 max_token_num_each_node: 20000