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/users.rs

@@ -89,12 +89,6 @@ where
     pub fn contains_key(&self, key: &K) -> bool {
         self.map.contains_key(key)
     }
-
-    /// Количество элементов
-    #[allow(dead_code)]
-    pub fn len(&self) -> usize {
-        self.map.len()
-    }
 }
 
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