Add test and release note for post list preview newline fix

[jkmassel]

Summary

• Fix the test assertion in testThatContentPreviewForDisplayTrimsLeadingAndTrailingNewlines to match actual firstParagraph() behavior
• Add release note for Post list preview: trim trailing newline in generated excerpt #25349

Test plan

• CI should pass on this branch
• PR Post list preview: trim trailing newline in generated excerpt #25349 is retargeted on top of this branch

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	31968
	Version	PR #25490
	Bundle ID	org.wordpress.alpha
	Commit	940e8da
	Installation URL	5q871btph9898

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	31968
	Version	PR #25490
	Bundle ID	com.jetpack.alpha
	Commit	940e8da
	Installation URL	0qs5amiq5ntv8

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[jkmassel]

I'm admin-merging this because it's just adding a release note after #25349