test: use example.test placeholder host, not .invalid

leak-guard bans the .invalid TLD (grouped with private .internal/.local/etc);
example.test is the RFC 6761 reserved test TLD — equally unreachable, not on
the ban list — so the leak-guard CI check passes.

tests/key_sources.rs

@@ -225,7 +225,7 @@ fn online_source_unconfigured_is_silent_no_op() {
 
 #[test]
 fn online_source_metadata() {
-    let src = OnlineSource::new("https://example.invalid/keys", "tok");
+    let src = OnlineSource::new("https://example.test/keys", "tok");
     assert_eq!(src.label(), "online");
     // No host-cert serving today — a no-op empty, no network touched.
     assert!(KeySource::host_certs(&src, None).is_empty());