refactor: complete large files/functions refactoring (Phase 6-7)

Phase 6: Refactor tdlib/client.rs ✅
- Extract update handlers to update_handlers.rs (302 lines, 8 functions)
- Extract message converter to message_converter.rs (250 lines, 6 functions)
- Extract chat helpers to chat_helpers.rs (149 lines, 3 functions)
- Result: client.rs 1259 → 599 lines (-52%)

Phase 7: Refactor tdlib/messages.rs ✅
- Create message_conversion.rs module (158 lines)
- Extract 6 helper functions:
  - extract_content_text() - content extraction (~80 lines)
  - extract_entities() - formatting extraction (~10 lines)
  - extract_sender_name() - sender name with API call (~15 lines)
  - extract_forward_info() - forward info (~12 lines)
  - extract_reply_info() - reply info (~15 lines)
  - extract_reactions() - reactions extraction (~26 lines)
- Result: convert_message() 150 → 57 lines (-62%)
- Result: messages.rs 850 → 757 lines (-11%)

Summary:
- ✅ All 4 large files refactored (100%)
- ✅ All 629 tests passing
- ✅ Category #2 "Large files/functions" COMPLETE
- ✅ Documentation updated (REFACTORING_OPPORTUNITIES.md, CONTEXT.md)

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

src/tdlib/mod.rs

@@ -1,12 +1,16 @@
 // Модули
 pub mod auth;
+mod chat_helpers; // Chat management helpers
 pub mod chats;
 pub mod client;
 mod client_impl; // Private module for trait implementation
+mod message_converter; // Message conversion utilities (for client.rs)
+mod message_conversion; // Message conversion utilities (for messages.rs)
 pub mod messages;
 pub mod reactions;
 pub mod r#trait;
 pub mod types;
+mod update_handlers; // Update handlers extracted from client
 pub mod users;
 
 // Экспорт основных типов