fix(notebooks): rewrite 04_vectorizers.ipynb with correct APIs

Fixed notebook to match actual RedisVL4j API:
- Removed broken Maven dependency loading (use local JARs instead)
- Fixed class names (use LangChain4JVectorizer, not OpenAIVectorizer)
- Fixed IndexSchema builder API (addTextField/addVectorField with consumers)
- Fixed VectorField imports (use VectorAlgorithm from Jedis)
- Fixed BaseVectorizer API (generateEmbedding/generateEmbeddingsBatch)
- Use UnifiedJedis instead of JedisPooled

Features demonstrated:
- HuggingFace Sentence Transformers (local, no API key)
- OpenAI via LangChain4j (text-embedding-ada-002)
- Cohere via LangChain4j (embed-english-v3.0)
- VoyageAI via LangChain4j (voyage-law-2)
- Custom vectorizer implementation
- Vector search with same query as Python ("That is a happy cat")

All examples verified with VectorizersNotebookIntegrationTest (7/7 passing).

Same models and data as Python version:
- Sentences: "That is a happy dog", "That is a happy person", "Today is a sunny day"
- Query validates semantic search (happy cat → happy dog)

Author: bsbodden