Fix GutenbergKit site media selection

[crazytonyli]

Description

Follow up review comments in #25855

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	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	33581
	Version	PR #25857
	Bundle ID	org.wordpress.alpha
	Commit	f8e7e19
	Installation URL	1fa5ki94et1v0

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This build has failures. Claude has analyzed them - check the build annotations for details.

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	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	33581
	Version	PR #25857
	Bundle ID	com.jetpack.alpha
	Commit	f8e7e19
	Installation URL	3gg9o16rduqsg

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[dcalhoun]

The changes look good to me. I performed regression testing in both GBK and GBM on Simple, Atomic, and self-hosted sites. I focused on selecting various file types from the Media Library. I did not encounter any issues.

[dcalhoun]

@crazytonyli question about the blog == %@ constraint this PR added to mapMediaIdsToMedia: Claude flagged it while working on a follow-up in #25866 and I'm not sure whether it's a real problem or not. Below is a summary. WDYT?

The fetch runs on ContextManager.shared.mainContext, but the predicate constrains on blog, which is whatever was passed to GutenbergMediaPickerHelper's initializer:

let context = ContextManager.shared.mainContext
let request = NSFetchRequest<NSManagedObject>(entityName: "Media")
request.predicate = NSPredicate(
    format: "blog == %@ AND mediaID IN %@",
    blog,
    mediaIds.map { NSNumber(value: $0) }
)

A Core Data predicate comparing a relationship against a managed object from a different context matches nothing, so if blog ever belongs to a context other than mainContext, the fetch silently returns no rows and the picker opens with no preselection.

Every path I traced passes a mainContext blog, though I didn't audit this exhaustively — so this may well be unreachable by construction. Is it? If GutenbergMediaPickerHelper is only ever handed a mainContext blog by design, this is a non-issue and I'll leave it alone.

I did try fetching on blog.managedObjectContext locally, but backed it out — SiteMediaCollectionViewController's fetched results controller is hardcoded to mainContext and compares selection by object identity, so that just moves the mismatch somewhere worse.

[crazytonyli]

I think we should be okay here. The theory is correct, and we can certainly make the code more robust by not using a Blog instance. But I believe the Blog instance used here is in the main context. It's a convention where the UI related types uses Core Data instance in the main context.

[dcalhoun]

Thank you for providing your perspective. Very helpful. 🙇🏻‍♂️