appservice: keep the crypto method set on the wrapped state store

The lazy store embedded appservice.StateStore, but the client's store is
also type-asserted to crypto.StateStore -- a wider interface -- and
NewCryptoHelper refuses to start when that assertion fails. Embedding the
narrower interface dropped GetEncryptionEvent and FindSharedRooms from the
concrete type, so the bot died on startup with "the client state store
must implement crypto.StateStore".

Embed both, assert both at compile time, and resolve GetEncryptionEvent
lazily like IsEncrypted since the crypto machine reads megolm rotation
settings through it.

internal/bot/statestore_test.go

@@ -4,13 +4,14 @@ import (
 	"context"
 	"net/http"
 	"net/http/httptest"
+	"path/filepath"
 	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
 
 	"maunium.net/go/mautrix"
-	"maunium.net/go/mautrix/appservice"
+	"maunium.net/go/mautrix/crypto/cryptohelper"
 	"maunium.net/go/mautrix/id"
 )
 
@@ -53,11 +54,26 @@ func (f *fakeHS) start(t *testing.T) *mautrix.Client {
 
 func newTestStore(t *testing.T, hs *fakeHS) *lazyStateStore {
 	t.Helper()
-	store := newLazyStateStore(mautrix.NewMemoryStateStore().(appservice.StateStore))
+	store := newLazyStateStore(mautrix.NewMemoryStateStore().(innerStateStore))
 	store.client = hs.start(t)
 	return store
 }
 
+// The store is handed to the crypto helper, which type-asserts it and refuses to
+// start if it comes up short. That assertion is the whole bot's startup path, so
+// pin it here rather than discovering it as a crash loop in prod.
+func TestStoreSatisfiesCryptoHelper(t *testing.T) {
+	store := newTestStore(t, &fakeHS{})
+
+	client := store.client
+	client.StateStore = store
+
+	dir := t.TempDir()
+	if _, err := cryptohelper.NewCryptoHelper(client, []byte("test"), filepath.Join(dir, "crypto.db")); err != nil {
+		t.Fatalf("crypto helper rejected the state store, so the bot would not start: %v", err)
+	}
+}
+
 const roomID = id.RoomID("!room:example.org")
 
 // The regression: a room configured as encrypted before the process started. No