Closes #56808

Problem:

This PR resolves an issue where embedded images in email views with multi-line <img> tags were not correctly replaced with inline data URIs.

Fix:

• The fix updates the replaceEmbeddedAttachments logic to handle multi-line tags, ensuring images are properly rendered in the final HTML.
• Added a new test case was added to verify that multi-line embedded images are correctly processed and replaced with data:image/png;base64,... URIs, and that no cid: references remain in the output.

Impact:

• No breaking changes.
• Regex is now safer and more robust for multi-line tags.