Fix RuntimeConfig validation for required topics

[TalkingJupiter]

Context

parseRuntimeConfigJson() reports a missing telemetry topic in its error branch, but RuntimeConfig::isValid() does not currently require telemetryTopic. That can let incomplete config pass validation.

Scope

Make runtime config validation match the fields the firmware actually requires.

Implementation notes

Review ESP32-Firmware/src/RuntimeConfig.cpp and decide which fields are required:

• configured
• enabled
• mac
• rack_id
• role
• topics.telemetry
• maybe topics.status
• maybe topics.event

Acceptance criteria

• Config without a required telemetry topic fails validation.
• Validation errors identify the missing/invalid field clearly.
• Valid config still parses successfully.
• If tests are added later, include valid config, missing telemetry topic, disabled device, unknown role, and missing rack ID cases.