Add fenced?/fenced= for code block nodes

[fukayatsu]

Summary

• Add fenced? getter and fenced= setter on code block nodes for the underlying comrak NodeCodeBlock.fenced boolean field
• The fenced field is already accepted as a required parameter in Node.new(:code_block, fenced: ...), but there were no getter/setter methods

Motivation

When walking the AST, it is currently impossible to distinguish between fenced code blocks (using ``` or ~~~) and indented code blocks (4 spaces), since both can have an empty fence_info string.

The current workaround is comparing source_position line counts against string_content line counts, but this is fragile (e.g., end_column == 0 edge cases when content follows the code block).

With fenced?, the distinction becomes trivial:

doc.walk do |node|
  next unless node.type == :code_block

  if node.fenced?
    # fenced code block (``` or ~~~)
  else
    # indented code block (4 spaces)
  end
end

Test plan

• Added tests for fenced? returning true on fenced code blocks
• Added tests for fenced? returning false on indented code blocks
• Added test for fenced= setter
• Added tests for fenced? raising TypeError on non-code-block nodes
• All 796 tests pass

[gjtorikian]

Can you add one more line to the test here which shows that once a code block is marked as fenced, converting the AST into HTML generates it as fenced?

[fukayatsu]

Added an HTML output assertion in 4bbbab5.

However, since the generated HTML for fenced and indented code blocks is identical (both render as <pre><code>...</code></pre>), there's no way to distinguish between the two in the HTML output. So the assertion only verifies that the code block is rendered correctly after setting fenced = true, rather than confirming it's specifically rendered as a fenced block.

Let me know if this is sufficient or if you had something else in mind!

[gjtorikian]

ugh, sorry, I was typing quickly in between tasks. I meant that I wanted to assert that backticks were appearing, and not spaces. I added this in cbd7b81.

[fukayatsu]

I see, that makes sense. Thank you!

[gjtorikian]

thanks!