fix: preserve chat history when switching with /code - #5892

Switching from ask mode with an empty /code command currently passes the model's edit format directly to cmd_chat_mode, which enables summarization and replaces the completed chat history. Route empty and whitespace-only /code commands through the existing code mode so they preserve the full history, matching /chat-mode code.

Add regression coverage for whole, diff, and udiff, including completed and current messages, empty and whitespace-only input, and /code with a prompt.

Validation:

• The six history-preservation cases fail on the original implementation and pass with this change.
• pytest tests/basic/test_commands.py tests/basic/test_coder.py -q: 114 passed, 38 subtests passed on Python 3.12.
• Pinned isort, Black, flake8, and codespell checks passed for both changed files.

Fixes #5878.