feat(mqtt): schema validation and pluggable payload deserializers

[mkjsix]

Sub-issue of #601.

Summary

Add a pluggable PayloadDeserializer interface and optional JSON Schema validation per topic, so that payloads are parsed, validated, and optionally rejected before reaching the SSE pipeline or MongoDB writer.

Motivation

The v1 module (#601) treats all payloads as opaque byte[]. In production, brokers emit payloads in various formats (JSON, plain text, binary Protobuf, Avro). Without a deserializer layer, every consumer must parse the raw bytes independently and handle malformed payloads individually.

Proposed design

PayloadDeserializer interface

public interface PayloadDeserializer<T> {
    T deserialize(byte[] payload, String topic) throws PayloadDeserializationException;
}

Built-in implementations:

• Utf8StringDeserializer (default — current behaviour)
• JsonNodeDeserializer — parse as Jackson JsonNode; invalid JSON routes to dead-letter
• ProtobufDeserializer — configurable .proto descriptor path
• AvroDeserializer — configurable Avro schema file or Schema Registry URL

Custom deserializers registered as @RegisterPlugin-annotated classes.

JSON Schema validation

plugins-args:
  mqtt-client:
    subscriptions:
      - topic: "sensors/#"
        qos: 1
        schema: "classpath:schemas/sensor-reading.json"   # JSON Schema draft-07
        on-validation-failure: "dead-letter"               # "dead-letter" | "drop" | "pass"

Invalid payloads are routed to the dead-letter log (see #607) with the validation error attached.

Scope

• PayloadDeserializer interface added to commons or as an internal mqtt SPI.
• Deserializer resolved per topic from config; falls back to Utf8StringDeserializer.
• JSON Schema validation using com.networknt:json-schema-validator.
• Unit tests for each built-in deserializer; integration test for validation + dead-letter routing.

Dependencies

• Blocked by feat: MQTT client module — bridge external broker events to HTTP/SSE/MongoDB #601.
• Dead-letter routing depends on feat(mqtt): dead-letter queue REST API #607 if on-validation-failure: dead-letter is used.