test: raise coverage across keydb, keydb_format, online, lib sources

Adds targeted unit tests for previously-uncovered branches: keydb load
failure classification (invalid-UTF-8 / unreadable path), zip extraction
success/no-cfg-member paths, write_atomic create/rename failure modes,
keydb_format hex-digit-parsing edge cases, iter_disc_entries, to_keydb_cfg
HC2 emission and empty-title/comment-field combinations, MultiSource::label,
and the online key-service client's SSRF-guard malformed-authority and
unresolvable-host paths, UK-as-string and VUK-derivation reply handling,
and get_fmts_indexes. Local line coverage moves from ~90.3% to ~92.1%.

tests/key_sources.rs

@@ -270,6 +270,15 @@ fn uk(b: u8) -> UnitKey {
     UnitKey::new(0, [b; 16])
 }
 
+/// `MultiSource` itself reports a fixed label, distinct from any inner
+/// source's own label — this is what a rip's source-attribution log shows
+/// when the composition, not one of its members, answered.
+#[test]
+fn multi_source_label_is_multi() {
+    let multi = MultiSource::new(vec![Box::new(ScriptedSource::new("A", Vec::new()))]);
+    assert_eq!(multi.label(), "multi");
+}
+
 #[test]
 fn multi_source_first_non_empty_wins_in_caller_order() {
     // Caller supplies [A, B]; A is non-empty so A's keys win.