Fix prepare-firmware.sh - redirect debug output to stderr

Fixed error where debug output was being passed as filename to esptool.py:

**Problem:** The find_file function echoed debug messages to stdout, which
were then captured by command substitution and passed as arguments to
esptool.py, causing "No such file or directory" errors with the debug text
as the filename.

**Root Cause:** In bash, command substitution $(command) captures ALL output
from the command, not just the intended return value. The function was
echoing both debug messages and the file path to stdout.

**Fix:**
- Redirect all debug output to stderr using >&2
- Only the actual file path goes to stdout
- Command substitution now captures just the file path
- Debug output is still visible to user but doesn't interfere with logic

The script will now correctly pass file paths to esptool.py while still
showing helpful debug information.

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

Author: Copilot

test/wokwi/prepare-firmware.sh

@@ -36,23 +36,24 @@ echo "Searching for required files in build directory..."
 echo ""
 
 # Function to find file with multiple possible locations
+# Debug output goes to stderr, only the file path goes to stdout
 find_file() {
     local file_desc=$1
     shift
     local found_file=""
 
-    echo "Looking for $file_desc:"
+    echo "Looking for $file_desc:" >&2
     for path in "$@"; do
-        echo "  Checking: $path"
+        echo "  Checking: $path" >&2
         if [ -f "$path" ]; then
             found_file="$path"
-            echo "  ✓ Found at: $path"
+            echo "  ✓ Found at: $path" >&2
             echo "$found_file"
             return 0
         fi
     done
 
-    echo "  ❌ Not found in any expected location"
+    echo "  ❌ Not found in any expected location" >&2
     return 1
 }