Add Gemini demo for LightRAG

This PR adds a Gemini-based direct demo for LightRAG, mirroring the
existing llamaindex direct demo.

Features:
- Uses gemini-2.0-flash for LLM inference
- Uses text-embedding-004 for embeddings
- Demonstrates naive, local, global, and hybrid query modes
- Includes required storage initialization
- Keeps a minimal, dependency-free setup

This serves as a reference example for users who want to use
LightRAG with Google Gemini without external frameworks.

examples/unofficial-sample/lightrag_gemini_demo.py

@@ -0,0 +1,89 @@
+import os
+import asyncio
+import nest_asyncio
+import numpy as np
+
+from lightrag import LightRAG, QueryParam
+from lightrag.llm.gemini import gemini_model_complete, gemini_embed
+from lightrag.utils import wrap_embedding_func_with_attrs
+
+nest_asyncio.apply()
+
+WORKING_DIR = "./rag_storage"
+GEMINI_API_KEY = os.environ.get("GEMINI_API_KEY", "your-api-key-here")
+
+if not os.path.exists(WORKING_DIR):
+    os.mkdir(WORKING_DIR)
+
+
+# --------------------------------------------------
+# LLM function
+# --------------------------------------------------
+async def llm_model_func(prompt, system_prompt=None, history_messages=[], **kwargs):
+    return await gemini_model_complete(
+        prompt,
+        system_prompt=system_prompt,
+        history_messages=history_messages,
+        api_key=GEMINI_API_KEY,
+        model_name="gemini-2.0-flash",
+        **kwargs
+    )
+
+# --------------------------------------------------
+# Embedding function
+# --------------------------------------------------
+@wrap_embedding_func_with_attrs(
+    embedding_dim=768,
+    max_token_size=2048,
+    model_name="models/text-embedding-004"
+)
+async def embedding_func(texts: list[str]) -> np.ndarray:
+    return await gemini_embed.func(
+        texts,
+        api_key=GEMINI_API_KEY,
+        model="models/text-embedding-004"
+    )
+
+# --------------------------------------------------
+# Initialize RAG
+# --------------------------------------------------
+async def initialize_rag():
+    rag = LightRAG(
+        working_dir=WORKING_DIR,
+        llm_model_func=llm_model_func,
+        embedding_func=embedding_func,
+        llm_model_name="gemini-2.0-flash",
+    )
+
+    # 🔑 REQUIRED
+    await rag.initialize_storages()
+    return rag
+
+
+# --------------------------------------------------
+# Main
+# --------------------------------------------------
+def main():
+    rag = asyncio.run(initialize_rag())
+
+    # Insert text
+    with open("./book.txt", "r", encoding="utf-8") as f:
+        rag.insert(f.read())
+
+    query = "What are the top themes?"
+
+    print("\nNaive Search:")
+    print(rag.query(query, param=QueryParam(mode="naive")))
+
+    print("\nLocal Search:")
+    print(rag.query(query, param=QueryParam(mode="local")))
+
+    print("\nGlobal Search:")
+    print(rag.query(query, param=QueryParam(mode="global")))
+
+    print("\nHybrid Search:")
+    print(rag.query(query, param=QueryParam(mode="hybrid")))
+
+
+if __name__ == "__main__":
+    main()