8. Feature: Add MQTT read-only adapter foundation

[smithaldon1]

Parent Epic

#206

Purpose

Implement the foundation for consuming configured MQTT sources into normalized FactoryEvents.

Acceptance Criteria

• Reads MQTT connection profiles.
• Supports broker URL, client ID, topic filters, QoS, TLS/auth references, and payload mapping.
• Supports Sparkplug-style JSON demo payloads as a first supported shape.
• Handles broker unavailable, malformed payload, and unmapped topic states clearly.
• Does not publish commands or perform writeback.

Implementation Order

Step 8. After OPC-UA adapter foundation unless ADR chooses another order.

Test Plan

Unit tests with fake MQTT messages plus optional local Mosquitto/Demo-Factory subscriber integration.