[12.x] Fix embedded image Content-ID inconsistency in cloned emails

[yinheli]

Problem

When using failover mailer configurations (e.g., Mailgun → AWS SES), embedded
images in emails fail to display after the email object is cloned for retry.

The issue occurs because the Content-ID referenced in HTML (<img src="cid:...">)
and the Content-ID in the MIME attachment become inconsistent after cloning.

Root Cause

Incorrect usage of the DataPart constructor in embed() and embedData() methods. The second parameter of DataPart is filename, not Content-ID.
DataPart generates its own Content-ID automatically.

• \Symfony\Component\Mime\Part\DataPart
• \Illuminate\Mail\Message

Reproduction

I've isolated a unit test to simulate the failover clone scenario. To reproduce the issue, please check out commit 23b3783.

Apology and Context

I apologize for not including a reproduction test in my previous PR #57606. This PR adds a failing test case and the corresponding fix.
Thank you for the patience and thorough review.

[taylorotwell]

I feel like we're changing a lot of code and logic just to fix what I would imagine is a much simpler bug? 👀 What is the minimum possible change to fix the issue?

[yinheli]

While fixing the misuse of the DataPart API, I found that the tests also needed adjustments. I thought it would be best to fix everything properly.