Enable the on-demand mOTA context for every classic ESP32 build

Move OTA_HEAP_CONTEXT from two hand-edited envs to a pre-script gated on
build.mcu == "esp32", so every classic ESP32 image gets it and none can drift.
S2/S3/C-series and nRF52 keep the .bss singleton: they have no equivalent
static-DRAM ceiling, so there a guaranteed-present workspace is the better
trade for what is ultimately a recovery path. It has to be a build flag rather
than a header default, because OTA_HEAP_CONTEXT must hold the same value in
every translation unit that sees OtaContext.h - OtaContext.cpp included - and a
header test on CONFIG_IDF_TARGET_ESP32 would depend on include order relative
to sdkconfig.h. The script defers to OTA_SHARED_COMPANION_QUEUE where a Full
Companion recipe already owns the storage.

Fix a gap this exposes in the roles that now use it. Only CLI entry points
acquire the context, so once it was released, a repeater with the temporary
radio profile up would no longer serve or announce its own firmware - LoRa OTA
would look enabled and silently do nothing. ota_service_temp_radio_context()
holds the workspace for exactly the temp-radio window and hands it back
outside it, which is where the saving was coming from anyway. Wired into the
repeater, room server and sensor loops; the Companion keeps its own
acquire-on-host-demand policy, since its context is borrowed from the offline
message queue and must not be taken speculatively.

Static DRAM, classic ESP32 (bytes occupied of a 124,580 region):
  Heltec_v2_repeater                   108,332 -> 70,732
  Heltec_v2_room_server                 83,716 -> 68,300
  Heltec_v2_companion_radio_ble        123,236 -> 107,772  (was failing on main)
  Tbeam_SX1262_repeater                 86,932 -> 71,468
  Tbeam_SX1262_repeater_observer_mqtt  121,132 -> 83,532   (was failing)

examples/simple_sensor/SensorMesh.cpp

@@ -5,6 +5,9 @@
 #include <helpers/LazyPersistence.h>
 #include <helpers/radiolib/RXPowerSaving.h>
 #include <helpers/UsbLogging.h>
+#if defined(ENABLE_OTA)
+#include <helpers/ota/OtaContext.h>
+#endif
 
 static uint32_t nextRadioApplyRetryDelay(uint8_t& failure_count) {
   uint8_t shift = failure_count < 5 ? failure_count : 5;
@@ -1343,6 +1346,9 @@ void SensorMesh::loop() {
           (unsigned long)retry_delay);
     }
   }
+#if defined(ENABLE_OTA) && OTA_DYNAMIC_CONTEXT
+  mesh::ota::ota_service_temp_radio_context(isTempRadioActive());
+#endif
 }
 
 bool SensorMesh::isMillisTimerDue(unsigned long timestamp) const {