Extract JSON payload builders and add format regression tests

[TalkingJupiter]

Context

The hello payload already shipped a malformed-JSON bug once (missing quote: "message_type":hello") — it compiled fine and would only have surfaced at the host. Nothing today verifies that the JSON the firmware emits is actually JSON.

The builders are currently untestable by structure:

• buildTelemetryJson() and appendTempArray() are static inside main.cpp
• the hello payload snprintf is inline in TelemetrySender::hello()

Scope

Extract the payload builders into their own compilation unit (e.g. TelemetryJson.h/.cpp) so tests can call them, then add Unity tests that feed the output back through ArduinoJson's parser.

Test cases

• Hello payload parses as JSON; message_type == "hello", mac matches input
• Telemetry JSON parses end to end
• NAN and the DS18B20 error sentinel 85.0 render as null in temperature arrays
• failoverDetails containing quotes / control characters is escaped and does not break the document
• Undersized output buffer truncates safely instead of overflowing

Acceptance criteria

• Builders live outside main.cpp and firmware behavior is unchanged.
• Every emitted payload shape has a parse-it-back test.
• The historical missing-quote bug class cannot recur silently.