fix: gateway pre-flight check could never actually fail

Verified live that after deploying the "fixed" bridge-bound pre-flight, it
still let a known-bad vmbr0+192.168.8.2 config straight through to a full
pct_create with no error. Root cause: the check used
`strings.Contains(pingOut, "REACHABLE")` against markers "REACHABLE" /
"UNREACHABLE" — but "UNREACHABLE" contains "REACHABLE" as a substring, so the
containment check was true for BOTH outcomes. The pre-flight was structurally
incapable of ever failing, regardless of the actual ping result.

Fixed with distinct, non-overlapping markers (PREFLIGHT_OK/PREFLIGHT_FAIL)
and exact-match comparison, pulled into a small gatewayPreflightPassed()
helper with a unit test asserting the exact historical bug case
("UNREACHABLE" must be false) so this bug class can't silently recur.

Re-verified live end-to-end: manually re-tested the exact ping command
(confirmed UNREACHABLE via vmbr0), and this was caught only by actually
running the check against production, not by reading the code.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/httpapi/pct_create_test.go

@@ -88,6 +88,32 @@ func TestJSONErrValidForNastyOutput(t *testing.T) {
 	}
 }
 
+// TestGatewayPreflightPassed guards the exact bug found live: "UNREACHABLE"
+// contains "REACHABLE" as a substring, so a strings.Contains(out,"REACHABLE")
+// check is true for BOTH outcomes and can never fail. Exact-match only.
+func TestGatewayPreflightPassed(t *testing.T) {
+	cases := []struct {
+		out  string
+		want bool
+	}{
+		{"PREFLIGHT_OK", true},
+		{"PREFLIGHT_OK\n", true},
+		{"  PREFLIGHT_OK  ", true},
+		{"PREFLIGHT_FAIL", false},
+		{"PREFLIGHT_FAIL\n", false},
+		{"", false},
+		{"some garbage output", false},
+		// the specific historical bug: a naive substring check on the old
+		// REACHABLE/UNREACHABLE markers would have called this true.
+		{"UNREACHABLE", false},
+	}
+	for _, c := range cases {
+		if got := gatewayPreflightPassed(c.out); got != c.want {
+			t.Errorf("gatewayPreflightPassed(%q) = %v, want %v", c.out, got, c.want)
+		}
+	}
+}
+
 func TestProvisionScript(t *testing.T) {
 	s := provisionScript([]string{"docker.io", "git"}, "echo hi > /root/x")
 	// Network/DNS gate must come before apt.