ragleap-rag (Java) v0.9.0

Adds MilvusBackend — a fifth vector backend alongside pgvector, FaissBackend, QdrantBackend, and WeaviateBackend. Unlike the other three REST-based backends, this one is explicitly NOT live-verified - no Milvus/Zilliz Cloud instance was available (self-hosting needs a 3-service stack: etcd, MinIO, milvus-standalone - judged too resource-risky on this VPS given existing CPU contention). Same honest caveat the Python MilvusBackend carries, but grounded in Milvus's real REST v2 API docs (fetched and read before writing code) rather than memory. No UUID indirection needed - Milvus's VarChar primary key accepts documentId:chunkIndex directly. 18 new tests via a local HTTP stub server, matching this codebase's existing pattern for testing unavailable providers. Published to Maven Central: io.github.antonyrag:ragleap-rag:0.9.0