Fix MarkdownRenderer re-rendering of code spans containing backticks

[gaoflow]

Problem

MarkdownRenderer.codespan always wraps the content in a single backtick pair:

return "`" + token["raw"] + "`"

When the code-span content itself contains backticks, re-rendering the AST produces Markdown that no longer round-trips:

• an inner backtick closes the span early, and
• a code span whose raw content is a single ` renders as ``` , which re-parses as an empty fenced code block.

This shows up through mistune's own round-trip oracle (to_html(text) vs to_html(reformat(text))):

from mistune import create_markdown
from mistune.renderers.markdown import MarkdownRenderer

to_html = create_markdown(escape=False)
reformat = create_markdown(renderer=MarkdownRenderer())

src = "``code with ` inside``\n"
assert to_html(reformat(src)) == to_html(src)  # fails on main

Fix

Size the delimiter to one more than the longest backtick run inside the content, and add one space of padding on each side when the content begins or ends with a backtick. That matches both CommonMark and mistune's own parse_codespan, which strips a single surrounding space back off (inline_parser.py: if code.startswith(" ") and code.endswith(" "): code = code[1:-1]).

For content with no backticks the delimiter stays a single backtick, so output is byte-for-byte identical to before; only backtick-containing spans change.

Tests

Added test_codespan_containing_backticks to TestMarkdownRendererRoundTrip, exercising single/double inner backticks plus leading, trailing, and surrounding backticks through the existing assert_round_trip helper. It fails before the change and passes after. Full suite: 985 passed; ruff check, ruff format, and mypy all clean.

I used an AI assistant to help investigate this under my direction, and I have reviewed and verified the change myself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.80%. Comparing base (74608f5) to head (f6827e8).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #450      +/-   ##
==========================================
+ Coverage   91.78%   91.80%   +0.01%     
==========================================
  Files          34       34              
  Lines        2653     2659       +6     
  Branches      433      434       +1     
==========================================
+ Hits         2435     2441       +6     
  Misses        146      146              
  Partials       72       72              

Flag	Coverage Δ
unittests	91.76% <100.00%> (+0.01%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud