Add vector_size parameter to QdrantSemanticCache and expose it through
the Cache facade as qdrant_semantic_cache_vector_size. This allows users
to use embedding models with dimensions other than the default 1536,
enabling cheaper/stronger models like Stella (1024d), bge-en-icl (4096d),
voyage, cohere, etc.
The parameter defaults to QDRANT_VECTOR_SIZE (env var or 1536) for
backward compatibility. When creating new collections, the configured
vector_size is used instead of the hardcoded constant.
Closes#9377
Zhenting Huang