Fix unterminated MyNodeInfo.pio_env when APP_ENV is 40+ chars (#11468)

strncpy does not null-terminate when the source fills the destination.
A PlatformIO environment name of 40 or more characters leaves pio_env
unterminated, nanopb aborts the whole MyInfo encode with 'unterminated
string', getFromRadio() returns 0 bytes forever, and the client app never
receives any config after want_config_id.

Clients that receive a redacted MyInfo (pio_env cleared before encode) are
unaffected, which makes the failure look client-specific when it is not.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/mesh/PhoneAPI.cpp

@@ -579,6 +579,9 @@ size_t PhoneAPI::getFromRadio(uint8_t *buf)
         // app not to send locations on our behalf.
         fromRadioScratch.which_payload_variant = meshtastic_FromRadio_my_info_tag;
         strncpy(myNodeInfo.pio_env, optstr(APP_ENV), sizeof(myNodeInfo.pio_env));
+        // strncpy does not terminate when the source fills the buffer; a 40+ char
+        // APP_ENV would make nanopb reject the MyInfo encode ("unterminated string").
+        myNodeInfo.pio_env[sizeof(myNodeInfo.pio_env) - 1] = '\0';
         myNodeInfo.nodedb_count = static_cast<uint16_t>(nodeDB->getNumMeshNodes());
         fromRadioScratch.my_info = myNodeInfo;
 #ifdef MESHTASTIC_PHONEAPI_ACCESS_CONTROL