Fix MarkdownRenderer corrupting email autolinks on round-trip

[gaoflow]

Summary

MarkdownRenderer corrupts email autolinks when re-rendering Markdown:

import marko
from marko.md_renderer import MarkdownRenderer

md = marko.Markdown(renderer=MarkdownRenderer)
md("<mail@x.com>")   # -> '<mailto:mail@x.com>\n'

Re-parsing that output no longer yields the original email link — <mailto:mail@x.com> is parsed as a URI autolink, so the rendered HTML link text becomes mailto:mail@x.com instead of mail@x.com. The MarkdownRenderer round-trip is broken for any email autolink. URI autolinks (<http://example.org>) round-trip correctly.

Cause

AutoLink.__init__ prepends "mailto:" to dest for emails, but preserves the original address in the children:

self.dest = match.group(1)
if re.match(patterns.email, self.dest):
    self.dest = "mailto:" + self.dest
self.children = [RawText(match.group(1))]

render_auto_link emitted the synthetic dest:

return f"<{element.dest}>"

so the mailto: prefix leaked into the Markdown output. (The HTML renderer correctly uses the children for the display text.)

Fix

Render the children — the original text — instead of dest. For URI autolinks the children equal dest, so they are unchanged:

return f"<{self.render_children(element)}>"

Verification

• Added test_markdown_renderer_email_autolink: asserts <mail@x.com> re-renders unchanged and that both an email and a URI autolink round-trip to equivalent HTML. It fails before the fix and passes after.
• Verified email autolinks with subdomains / +/-/. in the local part, explicit <mailto:...> URIs, and https autolinks all round-trip and are idempotent under re-rendering.
• Full suite: 1397 passed (was 1396). ruff and mypy clean on the changed files.

This pull request was prepared with the assistance of AI, under my direction and review.

[frostming]

MarkdownRenderer never guarantees round-trip consistency. It works more like a normalizer and SHOULD only ensure that the Markdown before and after conversion renders to the same HTML.

This can be verified:

before: https://spec.commonmark.org/dingus/?text=%3Cmail%40x.com%3E
after: https://spec.commonmark.org/dingus/?text=%3Cmailto%3Amail%40x.com%3E