Fix RTL_433 duplicate detection swallowing rapid transmissions with differing payloads

[Copilot]

isAduplicateSignal() in rtl_433_Callback() used only id + temperature_C as the dedup key, so any device lacking temperature_C (e.g. WH51 moisture sensor) would have all messages within the 3-second window silently dropped regardless of payload changes.

Changes

• Replace id + temperature_C hash with full-payload hash — iterates all JSON fields, hashing key+value for each, so any change in sensor data produces a distinct MQTTvalue
• Exclude volatile per-transmission fields — time, rssi, snr, noise_floor are skipped so retransmissions of the same physical signal still deduplicate correctly
• Order-independent accumulation — per-key hashes are summed rather than chained, avoiding sensitivity to JsonObject iteration order
• Consistent float serialization — uses serializeJson() per value instead of .as<String>() to avoid platform-dependent float formatting differences
• Key/value separator — 0xFF byte injected between key and value bytes prevents "ab"+"c" colliding with "a"+"bc"

// Before: only id + temperature_C — breaks for any sensor without temperature_C
unsigned long MQTTvalue = (int)RFrtl_433_ESPdata["id"] + round((float)RFrtl_433_ESPdata["temperature_C"]);

// After: stable hash over all non-volatile fields
unsigned long MQTTvalue = 0;
for (JsonPair kv : RFrtl_433_ESPdata) {
  // skip time, rssi, snr, noise_floor ...
  unsigned long kvHash = hash(key) + 0xFF_separator + hash(serializeJson(value));
  MQTTvalue += kvHash; // order-independent
}

Checklist:

• The pull request is done against the latest development branch
• Only one feature/fix was added per PR and the code change compiles without warnings
• I accept the DCO.

• Fixes RTL_433 swallows multiple rapid transmissions from the same device even if payload differs #2356