Integrate litellm for multi-provider LLM support

[KylinMountain]

Summary

• Replace openai + tiktoken with litellm for unified multi-provider LLM support (OpenAI, Azure, Gemini, etc.)
• Rename ChatGPT_API → llm_complete, ChatGPT_API_async → allm_complete (using litellm.acompletion for native async), ChatGPT_API_stream → llm_complete_stream
• Add backward compatibility: CHATGPT_API_KEY auto-aliased to OPENAI_API_KEY
• Remove tiktoken dependency; use litellm.token_counter instead

Test plan

• Set OPENAI_API_KEY and run existing PDF indexing — should behave identically to before
• Set CHATGPT_API_KEY (legacy) — should still work via alias
• Try a non-OpenAI provider (e.g. gemini/gemini-pro) by setting the appropriate env var

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