PositionModule::sendLostAndFoundText: use stack buffer, eliminate heap alloc (#10251)

* PositionModule::sendLostAndFoundText: use stack buffer, eliminate heap alloc

The lost-and-found message was built with an unnecessary heap allocation:

    char *message = new char[60];
    sprintf(message, "..."...);
    ...
    delete[] message;

Two problems:

1. **Buffer too small.** The format string expands with two %f (IEEE 754
   doubles), which `sprintf` prints with full precision — easily 15+
   digits each plus separators — so the actual rendered string can run
   40-50 characters before even considering the emoji (4 UTF-8 bytes)
   and the embedded BEL. A pathological lat/lon can overflow 60 bytes
   and corrupt heap metadata. Unbounded `sprintf` with no size check.

2. **Heap churn in a GPS callback.** This function is called from the
   position-update path which is already heap-sensitive. An infrequent
   60-byte transient alloc isn't catastrophic, but stack is trivially
   available here and removes the failure mode entirely.

Fix: replace with a 128-byte stack buffer and `snprintf` bounded by
`sizeof(message)`. Drop the matching `delete[]` since there's nothing
to delete.

Behavior is identical on the happy path; the overflow case now
truncates safely instead of scribbling over heap.

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* PositionModule.cpp: add trailing newline for clang-format

* Address Copilot review: cleaner snprintf size handling

Review feedback from @Copilot on PR #10251: the ternary-plus-static-cast
form mixed signed/unsigned types (int written vs. pb_size_t payload.size
vs. size_t sizeof(message)) and was harder to read than necessary.

Cleaner form:
  const size_t msg_len = std::min(static_cast<size_t>(written), sizeof(message) - 1);
  p->decoded.payload.size = msg_len;

Same behaviour (clamp to buffer-minus-NUL) with one explicit cast and
a size_t variable that names the meaning. Handles the encoding-error
path (written < 0) separately so no bad values leak into payload.size.

* Trunk

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

src/modules/PositionModule.cpp

@@ -492,15 +492,24 @@ void PositionModule::sendLostAndFoundText()
 {
     meshtastic_MeshPacket *p = allocDataPacket();
     p->to = NODENUM_BROADCAST;
-    char *message = new char[60];
-    sprintf(message, "🚨I'm lost! Lat / Lon: %f, %f\a", (lastGpsLatitude * 1e-7), (lastGpsLongitude * 1e-7));
+    char message[128];
+    int written = snprintf(message, sizeof(message), "🚨I'm lost! Lat / Lon: %f, %f\a", (lastGpsLatitude * 1e-7),
+                           (lastGpsLongitude * 1e-7));
     p->decoded.portnum = meshtastic_PortNum_TEXT_MESSAGE_APP;
     p->want_ack = false;
-    p->decoded.payload.size = strlen(message);
-    memcpy(p->decoded.payload.bytes, message, p->decoded.payload.size);
+    if (written < 0) {
+        // snprintf encoding error — send an empty payload rather than uninitialized bytes.
+        p->decoded.payload.size = 0;
+    } else {
+        // Clamp to buffer capacity (snprintf returns "would-have-written" which can exceed the buffer).
+        const size_t msg_len = std::min(static_cast<size_t>(written), sizeof(message) - 1);
+        p->decoded.payload.size = msg_len;
+        if (msg_len > 0) {
+            memcpy(p->decoded.payload.bytes, message, msg_len);
+        }
+    }
 
     service->sendToMesh(p, RX_SRC_LOCAL, true);
-    delete[] message;
 }
 
 // Helper: return imprecise (truncated + centered) lat/lon as int32 using current precision
@@ -580,4 +589,4 @@ void PositionModule::handleNewPosition()
     }
 }
 
-#endif
+#endif