chore: add mypy type checking for src/rag/

[prakja]

Summary

• Add mypy with config scoped to src/rag/ only (llm_manager, neet_rag, vector_store)
• Fix all 41 type errors across the 3 files (type annotations only, no behavioral changes)
• Key fixes: proper Optional types, return annotations, Union types for vector manager, targeted type: ignore for LangChain API stub mismatches
• src/processors/ and src/utils/ excluded via ignore_errors = true
• Run: mypy --config-file mypy.ini → 0 errors