Resolve v0.16 domain ids to names before comparing directories

x:Account.domainId is an internal id on v0.16 ("b"), not a domain name. A
pre-migration snapshot taken from a v0.15 instance records names
("smoke.test"), so the post-migration directory comparison compared ids
against names and would have reported every domain as having vanished -
a false alarm on the check whose whole job is proving nothing was lost.

The client now resolves them with x:Domain/query + x:Domain/get in a single
request, using a JMAP back-reference (RFC 8620 3.7). Confirmed against a
live 0.16.14 before being written:

    ["x:Domain/get", {"list":[{"name":"smoke.test","id":"b"}]}, "g"]

An id that can't be resolved is kept as-is - a domain that can't be named is
still a domain that exists - but a failure of the resolution call itself is
an error rather than a silent fallback, since quietly comparing ids against
names is precisely the bug being fixed.

Verified against the live migrated instance: the snapshot that reported
domains=[b] now reports domains=[smoke.test], matching what the
pre-migration snapshot recorded.

internal/validate/content_integrity_test.go

@@ -60,6 +60,13 @@ func fakeManagementServer(t *testing.T, accounts []map[string]any, mailboxesByEm
 		call := methodCalls[0].([]any)
 		name := call[0].(string)
 		switch name {
+		case "x:Domain/query":
+			json.NewEncoder(w).Encode(jmapEnvelope{MethodResponses: []any{
+				[]any{"x:Domain/query", map[string]any{"ids": []string{"d1"}}, "q"},
+				[]any{"x:Domain/get", map[string]any{"list": []map[string]any{
+					{"id": "d1", "name": "smoke.test"},
+				}}, "g"},
+			}})
 		case "x:Account/query":
 			ids := make([]string, len(accounts))
 			for i, a := range accounts {