refactor: Implement code review suggestions for emoji disable feature

- Move EMOJI_MAP to module level as DEFAULT_EMOJI_MAP for memory efficiency
- Add _parse_bool_env() helper function for cleaner boolean conversion
- Implement performance optimization with icon caching
- Improve error handling for unknown emojis with better fallback format
- Update tests to work with new module structure
- All functionality preserved and tests passing

Addresses feedback from PR review #3063

Co-Authored-By: João <joao@crewai.com>

tests/utilities/events/utils/test_console_formatter_emoji_disable.py

@@ -96,11 +96,11 @@ class TestConsoleFormatterEmojiDisable:
                 assert formatter._get_icon(emoji) == expected_text
 
     def test_unknown_emoji_fallback(self):
-        """Test that unknown emojis fall back to ASCII-only representation."""
+        """Test that unknown emojis fall back to proper representation."""
         with patch.dict(os.environ, {"CREWAI_DISABLE_EMOJIS": "true"}):
             formatter = ConsoleFormatter(verbose=True)
             result = formatter._get_icon("🦄")
-            assert result == ""
+            assert result == "[ICON:UNKNOWN]"
 
     @patch.dict(os.environ, {"CREWAI_DISABLE_EMOJIS": "true"})
     def test_crew_tree_creation_without_emojis(self):