Escape " in image titles (#586)

[agu2347]

Fixes #586.

Root cause

image() in src/mdformat/renderer/_context.py writes node.attrs["title"] straight into the (uri "title") form without escaping embedded double quotes:

title = node.attrs.get("title")
if title is not None:
    return f'![{description}]({uri} "{title}")'

link(), a few lines below, already handles this correctly:

title = title.replace('"', '\\"')
return f'[{text}]({uri} "{title}")'

An unescaped " in an image title closes the title early, producing malformed Markdown, and makes mdformat.text() non-idempotent on such input — reformatting the (already malformed) output a second time reparses it differently and even escapes the image's own brackets.

Reproduced exactly as described in the issue:

src = '![alt](img.png "a \\"quoted\\" title")'
mdformat.text(src)   # -> '![alt](img.png "a "quoted" title")\n'  (title truncated)
mdformat.text(out)   # -> '!\\[alt\\](img.png "a "quoted" title")\n'  (not idempotent)

Fix

Mirror link()'s escaping in image().

Tests

Added a link and image titles with embedded quotes case to tests/data/default_style.md covering both link and image titles with an embedded, escaped quote. Confirmed this case fails against the unfixed code — mdformat._cli.run() actually raises Could not format ...: Formatted Markdown renders to different HTML than input Markdown, mdformat's own consistency check catching the corruption — and passes after the fix. Full test suite: 4286 passed, 1 skipped, 0 failures.