Autolink URL backslashes are doubled on stringify

[ChristianMurphy]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Affected package

mdast-util-to-markdown 2.1.2, mdast-util-from-markdown 2.0.3, micromark 4.0.2

Steps to reproduce

<aa:\>

is stringified as:

<aa:\\>

the structure of the AST matches, but the URL stored on the link node gains
an extra backslash on the round-trip, so the rendered HTML href changes.

---

parse

<aa:\>

tree₁ has a link node with url: "aa:\\" (one backslash).

Stringify it:

<aa:\\>

tree₂ has a link node with url: "aa:\\\\" (two backslashes).

Rendered HTML differs:

<!-- tree1 -->
<p><a href="aa:%5C">aa:\</a></p>

<!-- tree2 -->
<p><a href="aa:%5C%5C">aa:\\</a></p>

Minimal reproducer:

import {fromMarkdown} from 'mdast-util-from-markdown'
import {toMarkdown} from 'mdast-util-to-markdown'

const tree1 = fromMarkdown('<aa:\\>')
const text2 = toMarkdown(tree1)    // '<aa:\\\\>\n'  (backslash doubled)

📓 per CommonMark §6.7, an autolink is delimited by <...> and its
content is interpreted literally — backslash escapes are not processed
inside autolinks. The serializer is escaping the backslash as if it were
inside regular text/link markup.

Larger real-world reproducers that collapse to the same root cause:
<https://example.com?find=\*>, <https://example.com/\[\>.

Actual behavior

Every backslash in the autolink URL is doubled on the round-trip.

Expected behavior

The round-trip preserves the URL. The serializer should emit the autolink
content verbatim (no backslash doubling).

Runtime

node v24.13.1

Package manager

npm 11.8.0

Operating system

Ubuntu (WSL2)

Build and bundle tools

No response