feat: Add handler-base library for NATS AI/ML services

- Handler base class with graceful shutdown and signal handling
- NATSClient with JetStream and msgpack serialization
- Pydantic Settings for environment configuration
- HealthServer for Kubernetes probes
- OpenTelemetry telemetry setup
- Service clients: STT, TTS, LLM, Embeddings, Reranker, Milvus

handler_base/clients/__init__.py

@@ -0,0 +1,18 @@
+"""
+Service client wrappers for AI/ML backends.
+"""
+from handler_base.clients.embeddings import EmbeddingsClient
+from handler_base.clients.reranker import RerankerClient
+from handler_base.clients.llm import LLMClient
+from handler_base.clients.tts import TTSClient
+from handler_base.clients.stt import STTClient
+from handler_base.clients.milvus import MilvusClient
+
+__all__ = [
+    "EmbeddingsClient",
+    "RerankerClient",
+    "LLMClient",
+    "TTSClient",
+    "STTClient",
+    "MilvusClient",
+]