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.

test/test_fuzz_packets/test_main.cpp

@@ -450,7 +450,8 @@ static meshtastic_AdminMessage fuzzAdminMessage()
         // manual bandwidth==0 path that used to SIGFPE the validator.
         r.set_config.which_payload_variant = meshtastic_Config_lora_tag;
         r.set_config.payload_variant.lora.region = (meshtastic_Config_LoRaConfig_RegionCode)rngRange(32);
-        r.set_config.payload_variant.lora.modem_preset = (meshtastic_Config_LoRaConfig_ModemPreset)rngRange(16);
+        r.set_config.payload_variant.lora.modem_preset =
+            (meshtastic_Config_LoRaConfig_ModemPreset)rngRange(_meshtastic_Config_LoRaConfig_ModemPreset_ARRAYSIZE);
         r.set_config.payload_variant.lora.use_preset = (rngRange(2) == 0);
         r.set_config.payload_variant.lora.bandwidth = rngRange(512); // includes 0
         r.set_config.payload_variant.lora.channel_num = rngNext();
@@ -553,7 +554,7 @@ static meshtastic_MeshBeacon fuzzBeacon()
         fuzzChannelSettings(b.offer_channel);
     b.offer_region = (meshtastic_Config_LoRaConfig_RegionCode)rngRange(32);
     b.has_offer_preset = (rngRange(2) == 0);
-    b.offer_preset = (meshtastic_Config_LoRaConfig_ModemPreset)rngRange(16);
+    b.offer_preset = (meshtastic_Config_LoRaConfig_ModemPreset)rngRange(_meshtastic_Config_LoRaConfig_ModemPreset_ARRAYSIZE);
     return b;
 }