Capture the pre-migration snapshot from 0.15.x, and stop claiming counts match when none were compared

Found by running preflight against a real Stalwart 0.15.5 in a VM. Two
defects, the second worse than the first.

1. AccountSnapshot could not read the version this tool migrates FROM.
   0.15.5 advertises no urn:stalwart:jmap capability and POST /api returns
   404 - the JMAP management API and x:Account are 0.16 features. 0.15.x
   exposes a REST API at GET /api/principal instead. So preflight's
   account-snapshot check warned and moved on, and every run against a real
   source instance had no "before" data at all.

   AccountSnapshot now dispatches on the capability the session document
   advertises - a positive signal, not an inference from a failed call -
   and internal/stalwartapi/principal.go implements the 0.15.x REST path,
   including its 1-based page/limit pagination so an install larger than
   one page isn't silently truncated.

2. With no "before" counts, the content-integrity comparison iterated an
   empty map, checked nothing, and reported "all message counts match".
   That is the strongest claim this tool makes - ARCHITECTURE 4.7 calls it
   the actual no-data-loss guarantee - made vacuously, and it would have
   passed on a migration that lost every message.

   The comparison now derives its account set from whatever the source
   could report, verifies every account and domain survived either way, and
   carries MessageCountsCompared so the report says plainly "MESSAGE COUNTS
   NOT COMPARED ... no-data-loss is NOT verified here" rather than implying
   otherwise.

What can and cannot be checked across the 0.15/0.16 boundary, now that a
real server has answered: 0.15.x has no per-mailbox message count at any
endpoint, and the impersonation login 0.16 offers returns 401 there, so
before/after message counts are impossible for the boundary migration this
tool exists for. Both versions do report per-account used quota (usedQuota
in 0.15's REST list, usedDiskQuota on 0.16's x:Account), so that is
captured on both sides. It is recorded and reported, not asserted on:
4.5 notes the 0.16 migration resets quotas to zero pending recalculation,
so comparing those bytes across the boundary would be a false alarm
generator.

Test servers across preflight, validate and stalwartapi now advertise
urn:stalwart:jmap, since they stand in for 0.16 instances and that
capability is what says so.

Verified end to end against the smoke VM: all nine preflight checks pass,
and the checkpoint records 2 accounts, 1 domain and per-account used quota
where it previously recorded nothing.

internal/validate/content_integrity_test.go

@@ -29,6 +29,15 @@ func fakeManagementServer(t *testing.T, accounts []map[string]any, mailboxesByEm
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.Method == http.MethodGet && r.URL.Path == "/.well-known/jmap" {
 			user, _, _ := r.BasicAuth()
+			if !strings.Contains(user, "%") {
+				// This instance is a migrated 0.16 one, which is what the
+				// urn:stalwart:jmap capability says.
+				json.NewEncoder(w).Encode(map[string]any{
+					"apiUrl":       apiURL,
+					"capabilities": map[string]any{"urn:ietf:params:jmap:core": map[string]any{}, "urn:stalwart:jmap": map[string]any{}},
+				})
+				return
+			}
 			target := strings.SplitN(user, "%", 2)[0]
 			if _, ok := mailboxesByEmail[target]; !ok {
 				w.WriteHeader(http.StatusForbidden)
@@ -166,3 +175,70 @@ func TestCompareContentIntegrityMultipleMailboxesPerAccount(t *testing.T) {
 		t.Errorf("mismatch = %+v, want Archive 199->200", m)
 	}
 }
+
+// The bug this guards against was found by running preflight against a real
+// Stalwart 0.15.5: it reports no per-mailbox counts, so the "before"
+// snapshot has none, and the comparison used to iterate that empty map,
+// check nothing, and report "all message counts match" - the strongest
+// claim this tool makes, made vacuously.
+func TestCompareContentIntegrityDoesNotClaimCountsMatchWhenSourceHadNone(t *testing.T) {
+	srv := fakeManagementServer(t,
+		[]map[string]any{{"id": "a1", "name": "[email protected]", "domainId": "smoke.test"}},
+		map[string][]map[string]any{"[email protected]": {{"name": "Inbox", "totalEmails": 3}}},
+	)
+	defer srv.Close()
+
+	// A 0.15.x-shaped snapshot: accounts and used-quota, no mailbox counts.
+	before := &checkpoint.PreflightSnapshot{
+		AccountCount: 1,
+		Domains:      []string{"smoke.test"},
+		UsedQuota:    map[string]int64{"[email protected]": 9207},
+	}
+	client := &stalwartapi.Client{BaseURL: srv.URL, Username: "admin", Password: "x"}
+	result, err := compareContentIntegrity(context.Background(), client, before)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.MessageCountsCompared {
+		t.Error("MessageCountsCompared = true, but the source snapshot had no counts")
+	}
+	if result.AccountsChecked != 1 {
+		t.Errorf("AccountsChecked = %d, want 1 - the account set must still be verified", result.AccountsChecked)
+	}
+	if strings.Contains(result.String(), "all message counts match") {
+		t.Errorf("report claims counts match when none were compared:\n%s", result)
+	}
+	if !strings.Contains(result.String(), "MESSAGE COUNTS NOT COMPARED") {
+		t.Errorf("report must say plainly that no-data-loss was not verified:\n%s", result)
+	}
+}
+
+// Presence checking still has to work on that path, or it would be no
+// better than the vacuous pass it replaced.
+func TestCompareContentIntegrityDetectsLostAccountWithoutCounts(t *testing.T) {
+	srv := fakeManagementServer(t,
+		[]map[string]any{{"id": "a1", "name": "[email protected]", "domainId": "smoke.test"}},
+		map[string][]map[string]any{"[email protected]": {{"name": "Inbox", "totalEmails": 3}}},
+	)
+	defer srv.Close()
+
+	before := &checkpoint.PreflightSnapshot{
+		AccountCount: 2,
+		Domains:      []string{"smoke.test", "gone.example"},
+		UsedQuota:    map[string]int64{"[email protected]": 9207, "[email protected]": 5380},
+	}
+	client := &stalwartapi.Client{BaseURL: srv.URL, Username: "admin", Password: "x"}
+	result, err := compareContentIntegrity(context.Background(), client, before)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.OK() {
+		t.Fatalf("want a failing result when an account and a domain vanished:\n%s", result)
+	}
+	if len(result.MissingAccounts) != 1 || result.MissingAccounts[0] != "[email protected]" {
+		t.Errorf("MissingAccounts = %v, want [[email protected]]", result.MissingAccounts)
+	}
+	if len(result.MissingDomains) != 1 || result.MissingDomains[0] != "gone.example" {
+		t.Errorf("MissingDomains = %v, want [gone.example]", result.MissingDomains)
+	}
+}