AdminModule: trust from == 0 only when the dispatcher reports a local origin (#12242)

AdminModule treats from == 0 as a local client, skipping the admin_key and
session-passkey checks. That trust rested entirely on every ingress path
dropping from == 0 (RadioLibInterface, MQTT downlink, UDP multicast and
frame injection all do). Make the handler check it itself:

- MeshModule gains a static currentRequestSource, set by callModules()
  alongside currentRequest, so modules can see the RxSource of the packet
  they are handling.
- AdminModule drops a from == 0 payload unless callModules() is dispatching
  that exact packet with RX_SRC_USER or RX_SRC_LOCAL. It fails closed for a
  radio source, for no dispatch context, and for a context that belongs to
  another packet. The drop is silent (no NAK to a forged sender).

Tests: AdminModuleTestShim::handleReceivedProtobuf now runs the handler
inside a dispatch context (default RX_SRC_USER, the phone client existing
suites stand in for), plus handleWithDispatchContext() for the negative
cases. test_admin_session_repro adds six cases: from == 0 via RX_SRC_RADIO,
with no context, and with another packet's context are rejected, both
directly and through MeshModule::callModules(); the phone-client controls
still apply the setter. With the gate removed, the four rejection cases fail.


Claude-Session: https://claude.ai/code/session_01Y1BYe4ahUHxyhw3nGTX67D

Co-authored-by: Claude <noreply@anthropic.com>

src/modules/AdminModule.cpp

@@ -135,6 +135,12 @@ bool AdminModule::handleReceivedProtobuf(const meshtastic_MeshPacket &mp, meshta
     if (mp.which_payload_variant != meshtastic_MeshPacket_decoded_tag) {
         return handled;
     }
+    // from == 0 is trusted as a local client below, so require callModules() to vouch for its origin.
+    if (mp.from == 0 &&
+        !(currentRequest == &mp && (currentRequestSource == RX_SRC_USER || currentRequestSource == RX_SRC_LOCAL))) {
+        LOG_WARN("Drop admin payload with from=0 that did not arrive from a local client");
+        return handled;
+    }
 #ifdef ARCH_PORTDUINO
     // Simulator only: honor exit_simulator unconditionally for the local client (from==0).
     // The from==0 branch below now covers pki_encrypted local packets too, but is_managed
@@ -161,8 +167,8 @@ bool AdminModule::handleReceivedProtobuf(const meshtastic_MeshPacket &mp, meshta
         LOG_TRACE("Allow admin response message");
     } else if (mp.from == 0) {
         // Local admin from a BLE/USB/TCP client. from == 0 cannot arrive from the
-        // mesh: RF drops packets without a sender (RadioLibInterface) and MQTT treats
-        // from == 0 as our own downlink and ignores it. Clients may set pki_encrypted
+        // mesh: RF, MQTT and UDP ingress drop it, and the source gate above rejects
+        // any from == 0 not dispatched as RX_SRC_USER/LOCAL. Clients may set pki_encrypted
         // on self-addressed admin (the python CLI does), so don't use it to reroute
         // local packets into the remote-PKC key check.
         //