scripts/discover-fortigate: validate by IP presence, not error-string match

Bug: FortiOS 7.x ana-gw replied to 'execute dhcp lease-list all' with
"Interface name 'all' does not exist." — my error-pattern grep didn't
include that phrase, so the script thought it got valid data, bailed
out of the retry loop, and handed empty/garbage to the parser, which
produced zero output with no error.

Fix: try the plain `execute dhcp lease-list` form first (works across
versions we've seen), fall back to the `all` variant only if the plain
form returns nothing. Validate acceptance by grepping for an actual
IP-shaped token — the parser needs IPs anyway, so "got real data"
and "has at least one IP" are equivalent conditions.

scripts/discover-fortigate.sh

@@ -47,9 +47,17 @@ USER="${FORTIGATE_SSH_USER:-admin}"
 # signal when the command returns empty. Let them flow to the caller.
 
 raw=""
-for cmd in 'execute dhcp lease-list all' 'execute dhcp lease-list'; do
-  raw=$(ssh -o StrictHostKeyChecking=accept-new "$TARGET" "$cmd" || true)
-  if [ -n "$raw" ] && ! grep -qiE 'unknown action|parse error|command fail' <<<"$raw"; then
+# Try the plain form first — works across FortiOS versions. `all` is
+# an interface name on some devices but errors with "Interface name
+# 'all' does not exist" on others (including 7.x ana-gw). Fall back
+# to `all` only if the plain form returns no lease rows.
+for cmd in 'execute dhcp lease-list' 'execute dhcp lease-list all'; do
+  out=$(ssh -o StrictHostKeyChecking=accept-new "$TARGET" "$cmd" || true)
+  # Accept only output that actually contains at least one IP-like token.
+  # This filters out "Interface name X does not exist" and similar noise
+  # without needing to enumerate every FortiOS error phrase.
+  if grep -qE '([0-9]+\.){3}[0-9]+' <<<"$out"; then
+    raw="$out"
     break
   fi
 done