When a code block ended without a trailing newline after the closing
\`\`\`, two bugs occurred in flush_incomplete():
1. The closing \`\`\` was included as part of the code block content
(displayed with syntax highlighting)
2. The same \`\`\` was then emitted again as literal text because
current_line was not cleared after being pushed to block_buffer
The fix:
- Check if current_line is the closing fence before adding to block_buffer
- Always clear current_line after processing in the CodeBlock case
Added two tests:
- test_code_fence_after_blank_line: code fence with trailing newline
- test_code_fence_no_trailing_newline: code fence without trailing newline