Add Unity unit tests for RuntimeConfig parsing and validation

[TalkingJupiter]

Context

parseRuntimeConfigJson() is the choke point every host config flows through, and it currently has zero coverage. It is also the place where the #6 validation gap and the #11 schema decisions live — writing these tests forces both.

Scope

On-device Unity tests under ESP32-Firmware/test/test_runtime_config/, run with pio test -e esp32_a over USB. On-device (not native) because RuntimeConfig uses Arduino String; native execution would need ArduinoFake or a std::string refactor, which is not worth it yet.

Test cases

• Valid full config → accepted, every field populated in the struct
• Garbage / truncated JSON → rejected, parse error in err
• message_type != "config" → rejected
• configured: false → rejected with "Device is not configured"
• enabled: false → rejected — documents the current remote-disable dead end; this test's expectation flips when Fix RuntimeConfig validation for required topics #6 is fixed
• Missing rack_id → rejected
• role: "controllerA" → rejected as Unknown — codifies the role-vocabulary decision from Align config schema between firmware and host provisioning example #11
• Missing topics.telemetry → currently passes validation, should fail — write it expecting failure, watch it fail, fix isValid(), watch it pass (this is the Fix RuntimeConfig validation for required topics #6 bug, test-first)
• parseRole / roleToString round-trip for Primary, Standby, Unknown

Acceptance criteria

• pio test -e esp32_a runs the suite on hardware and passes.
• The missing-telemetry-topic case fails until Fix RuntimeConfig validation for required topics #6 is fixed, then passes.
• Error strings are asserted, not just the boolean result.

Relates to #6, #11.