fix(markdown): preserve image URL in citation conversion

[devteamaegis]

Bug

DefaultMarkdownGenerator.convert_links_to_citations converts ![alt](url) image links to ![alt⟨n⟩] — dropping the (url) part. This is invalid Markdown: images without a URL render as broken empty brackets rather than the actual image.

Before (broken):

Input:  ![Logo](https://example.com/logo.png)
Output: ![Logo⟨1⟩]        ← no URL → broken image

After (fixed):

Input:  ![Logo](https://example.com/logo.png)
Output: ![Logo⟨1⟩](https://example.com/logo.png)  ← image renders + citation number

Root cause

The citation conversion for images only rebuilt the ![text⟨n⟩] part without the trailing (url):

# Before
else f"![{text}⟨{num}⟩]"

Regular hyperlinks [text](url) → text⟨n⟩ correctly omit the URL (it goes to the References section). But images need their URL kept inline because:

1. Markdown does not support reference-style images in all renderers.
2. The visual image must remain intact — users can't "look up" an image by citation number.

Fix

One-character change: add ({url}) back to the image output:

# After
else f"![{text}⟨{num}⟩]({url})"