fix: restore markdown history summarization helper

[MobiusL00p]

Summary

• restore ChatSummary.summarize_chat_history_markdown() so aider/history.py's CLI entrypoint no longer calls a missing method
• parse markdown chat history back into user/assistant messages while skipping headings, blockquotes, and slash-command markers
• add a regression test covering the markdown parsing path and keep the updated history tests clean under basedpyright

Verification

• python -m pytest -q tests/basic/test_history.py
• python -c "from aider.history import ChatSummary; from unittest import mock; m=mock.Mock(); m.token_count=lambda msg: 1; m.info={'max_input_tokens':4096}; s=ChatSummary([m], max_tokens=100); s.summarize=lambda messages: messages; print(s.summarize_chat_history_markdown('# aider chat\n#### Q\nA\n'))"