Fix run-simulator.sh to check for firmware-combined.bin

Fixed error where run-simulator.sh was checking for firmware.bin instead of the combined firmware image:

**Problem:** After switching to combined firmware approach, run-simulator.sh still checked for "firmware.bin" which no longer exists. The prepare-firmware.sh script now creates "firmware-combined.bin" instead.

**Fix:**
- Updated firmware existence check in run-simulator.sh
- Changed from "firmware.bin" to "firmware-combined.bin"
- Error message updated to reflect the correct filename

The simulator will now correctly detect the combined firmware image before starting.

Co-authored-by: softhack007 <91616163+softhack007@users.noreply.github.com>

Author: Copilot

test/wokwi/run-simulator.sh

@@ -9,9 +9,9 @@ WOKWI_TIMEOUT=${WOKWI_TIMEOUT:-300}
 
 cd "$SCRIPT_DIR"
 
-# Check if firmware exists
-if [ ! -f "firmware.bin" ]; then
-    echo "Error: firmware.bin not found in $SCRIPT_DIR"
+# Check if combined firmware exists
+if [ ! -f "firmware-combined.bin" ]; then
+    echo "Error: firmware-combined.bin not found in $SCRIPT_DIR"
     echo "Please run prepare-firmware.sh first"
     exit 1
 fi