feat(send): implement CopyMessage

CopyMessage re-sends a message's content as a new, unlinked message via a
forward with the author dropped (the MTProto equivalent of Bot API copyMessage).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ernado

copy.go

@@ -0,0 +1,31 @@
+package botapi
+
+import "context"
+
+// CopyMessage copies a message: it re-sends the message's content to another
+// chat as a new message, without the "forwarded from" header. Unlike
+// ForwardMessage, the result is not linked to the original.
+func (b *Bot) CopyMessage(ctx context.Context, to, from ChatID, messageID int, opts ...SendOption) (*Message, error) {
+	var cfg sendConfig
+	for _, o := range opts {
+		o(&cfg)
+	}
+
+	toPeer, err := b.resolveInputPeer(ctx, to)
+	if err != nil {
+		return nil, err
+	}
+	fromPeer, err := b.resolveInputPeer(ctx, from)
+	if err != nil {
+		return nil, err
+	}
+
+	builder := &b.sender.To(toPeer).Builder
+	builder, err = b.applySendConfig(builder, cfg)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err := builder.ForwardIDs(fromPeer, messageID).DropAuthor().Send(ctx)
+	return b.sentMessage(ctx, toPeer, resp, err)
+}