Move PostHelper to app target

[kean]

Closes https://linear.app/a8c/issue/CMM-1271/convert-postshelper-to-swift

In #25301, I converted the ReaderPost aspects to Swift as it'll likely stay in production for a while. There is no value converting the rest as the PostHelper as it's the first area of Core Data being replaced with Rust.

[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	31272
	Version	PR #25321
	Bundle ID	com.jetpack.alpha
	Commit	eb2ef81
	Installation URL	78e6ueg1iivag

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 WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	31272
	Version	PR #25321
	Bundle ID	org.wordpress.alpha
	Commit	eb2ef81
	Installation URL	309s5epln3uf8

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

[wpmobilebot]

🤖 Build Failure Analysis

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud