refactor: move omni_llm.py to entrypoints directory

- Move vllm_omni/core/omni_llm.py to vllm_omni/entrypoints/omni_llm.py
- Update all import statements across the codebase to reflect new location
- Fix relative imports within the moved file
- Maintain functionality while improving code organization
- All imports and functionality tested and working correctly

This change better reflects that OmniLLM and AsyncOmniLLM are the main entry points
for vLLM-omni functionality, rather than core implementation details.

examples/basic/simple_usage.py

@@ -7,7 +7,7 @@ with a single AR (Autoregressive) stage.
 """
 
 import asyncio
-from vllm_omni.core.omni_llm import OmniLLM, AsyncOmniLLM
+from vllm_omni.entrypoints.omni_llm import OmniLLM, AsyncOmniLLM
 from vllm_omni.config import create_ar_stage_config