Add MEDIUM_TURBO modem preset (#10988)

* Protobufs

* Wire up MEDIUM_TURBO modem preset

MEDIUM_TURBO (500 kHz, SF9, CR 4/5) already existed in the protobuf enum but
was never wired into firmware, so selecting it silently fell through to the
LONG_FAST default and rendered an "Invalid" display name.

Add its bw/sf/cr mapping (modemPresetToParams), display name (MediumTurbo/MedT),
PRESETS_STD membership (standard regions only — 500 kHz does not fit EU868's
250 kHz band, so it stays out of PRESETS_EU_868 and is rejected/clamped there),
and the MEDIUM SNR-grading bucket. Includes positive coverage in test_radio,
EU868-reject + US-accept coverage in test_admin_radio and test_mesh_beacon,
the STD preset count 9->10, an extended fuzz range, and the client-spec doc.

* Address review feedback on MEDIUM_TURBO tests

- test_mesh_beacon: assert has_mesh_beacon before checking the invalid preset was
  cleared, so the EU868-cleared test can't pass on a dropped message (matches the
  existing SHORT_TURBO test).
- test_fuzz_packets: draw modem presets from _ModemPreset_ARRAYSIZE instead of a
  hard-coded 17 so the fuzz range tracks future enum additions automatically.

src/DisplayFormatters.cpp

@@ -27,6 +27,9 @@ const char *DisplayFormatters::getModemPresetDisplayName(meshtastic_Config_LoRaC
     case PRESET(MEDIUM_FAST):
         return useShortName ? "MedF" : "MediumFast";
         break;
+    case PRESET(MEDIUM_TURBO):
+        return useShortName ? "MedT" : "MediumTurbo";
+        break;
     case PRESET(LONG_SLOW):
         return useShortName ? "LongS" : "LongSlow";
         break;