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_radio/test_main.cpp

@@ -195,6 +195,47 @@ static void test_applyModemConfig_customCodingRateLowerThanPreset()
     TEST_ASSERT_EQUAL_UINT8(8, testRadio->getCr());
 }
 
+// MEDIUM_TURBO performs like MEDIUM_FAST (sf=9, cr=5) but at 500 kHz. Verify the params resolve.
+static void test_applyModemConfig_mediumTurbo()
+{
+    config.lora = meshtastic_Config_LoRaConfig_init_zero;
+    config.lora.region = meshtastic_Config_LoRaConfig_RegionCode_US;
+    config.lora.use_preset = true;
+    config.lora.modem_preset = meshtastic_Config_LoRaConfig_ModemPreset_MEDIUM_TURBO;
+
+    testRadio->reconfigure();
+
+    TEST_ASSERT_EQUAL_UINT8(5, testRadio->getCr());
+    TEST_ASSERT_EQUAL_UINT8(9, testRadio->getSf());
+    TEST_ASSERT_FLOAT_WITHIN(0.01f, 500.0f, testRadio->getBw());
+}
+
+// MEDIUM_TURBO is a 500 kHz preset, so it is invalid for EU_868 and must clamp to the region default.
+static void test_clampConfigLora_mediumTurboInvalidForEU868()
+{
+    meshtastic_Config_LoRaConfig cfg = meshtastic_Config_LoRaConfig_init_zero;
+    cfg.use_preset = true;
+    cfg.region = meshtastic_Config_LoRaConfig_RegionCode_EU_868;
+    cfg.modem_preset = meshtastic_Config_LoRaConfig_ModemPreset_MEDIUM_TURBO;
+
+    RadioInterface::clampConfigLora(cfg);
+
+    TEST_ASSERT_EQUAL(meshtastic_Config_LoRaConfig_ModemPreset_LONG_FAST, cfg.modem_preset);
+}
+
+// MEDIUM_TURBO is valid for US (PROFILE_STD) and must be left unchanged.
+static void test_clampConfigLora_mediumTurboValidForUS()
+{
+    meshtastic_Config_LoRaConfig cfg = meshtastic_Config_LoRaConfig_init_zero;
+    cfg.use_preset = true;
+    cfg.region = meshtastic_Config_LoRaConfig_RegionCode_US;
+    cfg.modem_preset = meshtastic_Config_LoRaConfig_ModemPreset_MEDIUM_TURBO;
+
+    RadioInterface::clampConfigLora(cfg);
+
+    TEST_ASSERT_EQUAL(meshtastic_Config_LoRaConfig_ModemPreset_MEDIUM_TURBO, cfg.modem_preset);
+}
+
 // -----------------------------------------------------------------------
 // getRegionPresetMap() - region->valid-preset map sent to clients during want_config
 // -----------------------------------------------------------------------
@@ -317,6 +358,9 @@ void setup()
     RUN_TEST(test_applyModemConfig_codingRateMatchesPreset);
     RUN_TEST(test_applyModemConfig_customCodingRateHigherThanPreset);
     RUN_TEST(test_applyModemConfig_customCodingRateLowerThanPreset);
+    RUN_TEST(test_applyModemConfig_mediumTurbo);
+    RUN_TEST(test_clampConfigLora_mediumTurboInvalidForEU868);
+    RUN_TEST(test_clampConfigLora_mediumTurboValidForUS);
     RUN_TEST(test_regionPresetMap_coversAllRegionsWithinBounds);
     RUN_TEST(test_regionPresetMap_matchesRegionTable);
     exit(UNITY_END());