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/auth.rs

@@ -6,7 +6,6 @@ use tdlib_rs::functions;
 /// Отслеживает текущий этап аутентификации пользователя,
 /// от инициализации TDLib до полной авторизации.
 #[derive(Debug, Clone, PartialEq)]
-#[allow(dead_code)]
 pub enum AuthState {
     /// Ожидание параметров TDLib (начальное состояние).
     WaitTdlibParameters,