[PARENT] Test coverage for the MQTT provisioning and telemetry path

[TalkingJupiter]

Context

The repo currently has no test infrastructure at all. The dynamic-config work (hello → config → telemetry) has stabilized enough that its logic is worth pinning down — especially since the config parser is where the #6 validation fix and the #11 schema decisions will land, and a payload-format bug (invalid hello JSON) already slipped through once.

Plan

Three tiers, cheapest and highest-value first:

• Add Unity unit tests for RuntimeConfig parsing and validation #14 — Unity unit tests for RuntimeConfig parsing/validation (on-device, pio test)
• Extract JSON payload builders and add format regression tests #15 — Extract JSON payload builders from main.cpp / TelemetrySender and add parse-it-back format tests
• Add broker-level integration test harness (pytest + paho-mqtt) #16 — pytest + paho-mqtt integration harness that plays host against a bench device

Deliberately out of scope: Ethernet/W5500 bring-up (manual hardware checklist in #3), PubSubClient internals (library code), RoleManager/RelayControl (unused until #12).

Order & rationale

Tier 1 first — it forces the #6 fix and the #11 role/MAC decisions while writing it. Tier 2 is a small testability refactor with a proven bug class behind it. Tier 3 doubles as the executable contract the future Radxa provisioning service must satisfy.

Relates to #3, #6, #11.