text: Fix images inside tags not showing

[Moulberry]

Description

Fix images inside <a> tags not showing.
Previously, the code for <a> used merge_child_text. merge_child_text did not merge images, so images were lost. The new code merges without losing the images, while ensuring the images are placed in the correct location within the text.

How to Test

Edit README.md to contain

<a href="https://longbridge.com/desktop">
<img width="1763" alt="Image" src="https://github.com/user-attachments/assets/e1ecb9c3-2dd3-431e-bd97-5a819c30e551" />
</a>

And then open the "Introduction" story.

Before this change, the image is not visible.
After this change, the image correctly shows.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)

[huacnlee]

Thanks