chore: remove dead code and unnecessary allow(dead_code) attributes

Cleaned up warnings by removing unused code:
- Removed unused format_timestamp() function from utils.rs
- Removed unused len() method from LruCache
- Removed unused date field from ForwardInfo struct
- Removed unnecessary #[allow(dead_code)] attributes from:
  * AuthState enum (actually used)
  * ChatInfo struct (actually used)
  * TdClient impl block (actually used)

This reduces code noise and makes real warnings more visible.

Changes:
- 20 lines removed
- 1 line added
- 6 files changed

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/tdlib/types.rs

@@ -3,7 +3,6 @@ use tdlib_rs::types::TextEntity;
 use crate::types::{ChatId, MessageId};
 
 #[derive(Debug, Clone)]
-#[allow(dead_code)]
 pub struct ChatInfo {
     pub id: ChatId,
     pub title: String,
@@ -41,9 +40,6 @@ pub struct ReplyInfo {
 pub struct ForwardInfo {
     /// Имя оригинального отправителя
     pub sender_name: String,
-    /// Дата оригинального сообщения (для будущего использования)
-    #[allow(dead_code)]
-    pub date: i32,
 }
 
 /// Информация о реакции на сообщение