botapi: getChatMemberCount, getChatMember, and a fuller getChat

- getChatMemberCount: channel/supergroup participant count (numeric chat_id).
- getChatMember: resolves a member via GetParticipant, projected to a Bot API
  ChatMember (creator/administrator/restricted/member/left/kicked with the
  matching rights); a user simply not in an accessible chat returns "left".
- getChat now uses the full channel view and adds permissions (from the default
  restrictions), slow_mode_delay, linked_chat_id and pinned_message.

Channel-only methods reject user chat_ids; private chats the bot cannot access
return CHAT_NOT_FOUND.

internal/domain/botapi_chat.go

@@ -15,4 +15,18 @@ type BotAPIChat struct {
 	Verified    bool
 	Scam        bool
 	Fake        bool
+
+	// Channel/supergroup only, from the full view.
+	SlowModeDelay      int
+	LinkedChatID       int64                // domain channel id; the projection applies the Bot API encoding
+	Permissions        *ChannelBannedRights // default restrictions; nil for a user chat
+	PinnedMessage      *Message
+	PinnedMessageUsers []User
+}
+
+// BotAPIChatMember is a resolved chat member for the Bot API getChatMember
+// method.
+type BotAPIChatMember struct {
+	User   User
+	Member ChannelMember
 }