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)

src/helpers/ota/OtaContext.h

@@ -681,6 +681,22 @@ uint8_t ota_hop_limit();
 void ota_set_context_storage(void* owner, OtaContext* (*acquire)(void*),
                              void (*release)(void*));
 void ota_release_context_if_idle(bool temporary_radio_active);
+
+// Loop helper for roles whose LoRa OTA only runs under the temporary radio
+// profile (repeater, room server, sensor). Nothing else acquires the context
+// on their behalf: without this, serving and announcing would stop the moment
+// the context was released, because only CLI entry points acquire. Holding it
+// for the temp-radio window keeps behaviour identical to a permanent context,
+// and the node is outside that window nearly all the time, which is where the
+// saving comes from. Not for the Companion, whose context is borrowed from the
+// offline message queue and must only be taken on explicit host demand.
+inline void ota_service_temp_radio_context(bool temporary_radio_active) {
+  if (temporary_radio_active) {
+    ota_acquire_context(nullptr, 0);   // failure is reported at the CLI entry points
+  } else {
+    ota_release_context_if_idle(false);
+  }
+}
 #endif
 
 } // namespace ota