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/messages.rs

@@ -720,7 +720,6 @@ impl MessageManager {
             if let tdlib_rs::enums::MessageOrigin::User(origin_user) = &fi.origin {
                 Some(ForwardInfo {
                     sender_name: format!("User {}", origin_user.sender_user_id),
-                    date: fi.date,
                 })
             } else {
                 None