(new): support Bot API 10.3

(fix): finalize v2 contracts
(tests): cover v2 migration
(doc): prepare release guidance

tgrich/errors.go

@@ -3,6 +3,8 @@ package tgrich
 import "errors"
 
 var (
+	// ErrRichInvalidButton indicates an invalid rich-button action, label, style, or row.
+	ErrRichInvalidButton = errors.New("invalid rich button")
 	// ErrRichTextTooLong indicates that rich-message text exceeds 32768 UTF-8 characters.
 	ErrRichTextTooLong = errors.New("rich text too long")
 	// ErrRichTooManyBlocks indicates that a rich message exceeds 500 blocks and counted nested items.