Add MQTT read-only adapter foundation

[smithaldon1]

Closes #213

Summary

• Add an ingestion-layer MQTT read-only adapter foundation that loads enabled MQTT ProtocolConnectionProfile records from the local profile store.
• Consume configured topic filters through an injectable MqttMessageSource; no command publishing, writeback, arbitrary topic discovery, or broker runtime is added.
• Support broker URL, client ID, topic filters, QoS, TLS flag, and auth/certificate references through the existing profile contract passed to the message source.
• Normalize Sparkplug-style JSON demo payload metrics into process.measurement.recorded FactoryEvents with source metadata for connection profile ID, topic, metric name, adapter name, and poll index.
• Add clear adapter errors for invalid profile stores, unavailable brokers, malformed payloads, and unmapped topics.

Test Evidence

• .venv/bin/python -m pytest services/ingestion/tests/test_mqtt_read_only_adapter.py - passed, 11 tests.
• make test-integration - passed, 85 tests.
• make lint - passed, Ruff found no issues after import formatting and line wrapping.
• make typecheck - passed, compileall completed across packages and services.
• make test - passed, 257 tests.
• make docs - passed; docs are Markdown-only and no checker is configured yet.
• git diff --check and git diff --cached --check - passed.

Docs Impact

• Updated services/ingestion/README.md with the MQTT read-only adapter foundation, Sparkplug-style JSON payload shape, source metadata behavior, safety boundary, and focused test command.
• Updated docs/LEARNING_LOG.md with what changed, why it matters, how the data flows, how to test it, and the next learning step.

Follow-up Work

• Add a real broker-backed message source once connector runtime orchestration and secret resolution are defined.
• Add explicit tag/source mapping lookup so payload mappings can be governed by mapping_reference instead of adapter defaults and payload context.
• Keep publish commands, writeback, arbitrary topic discovery, and UI ingestion controls out of this adapter until dedicated issues and ADR updates define those boundaries.

[smithaldon1]

Reviewed and approved for merge