mtproto,rpc: support Android legacy startup

(cherry picked from commit 06b6ae2bb2f90bd7cc1d6a8404a82aff507e6821)

internal/rpc/compat.go

@@ -0,0 +1,44 @@
+package rpc
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"go.uber.org/zap"
+
+	"github.com/gotd/td/bin"
+)
+
+// dispatchCompat handles explicitly allowlisted legacy TL constructors that are
+// still emitted by supported clients but are absent from gotd's pinned schema.
+func (r *Router) dispatchCompat(ctx context.Context, b *bin.Buffer, id uint32) (bin.Encoder, bool, error) {
+	start := time.Now()
+	var (
+		enc  bin.Encoder
+		name string
+		err  error
+	)
+
+	switch id {
+	case legacyLangpackGetLanguagesTypeID:
+		name = "langpack.getLanguages#800fd57d"
+		enc, err = r.handleLegacyLangpackGetLanguages(ctx, b)
+	default:
+		return nil, false, nil
+	}
+
+	fields := append([]zap.Field{
+		zap.String("method", name),
+		zap.String("type_id", fmt.Sprintf("%#x", id)),
+		zap.Bool("compat", true),
+		zap.Duration("dur", time.Since(start)),
+	}, r.contextLogFields(ctx)...)
+	if err != nil {
+		fields = append(fields, zap.Error(err))
+		r.log.Info("RPC compat handled", fields...)
+	} else {
+		r.log.Debug("RPC compat handled", fields...)
+	}
+	return enc, true, err
+}