fix: Resolve iOS retain cycle in EditorViewController async flow

[dcalhoun]

What?

Fixes a retain cycle that prevents EditorViewController from being deallocated when using the async dependency-loading flow.

Why?

Fix CMM-1282.

When opening and closing the editor multiple times without pre-loaded dependencies, each EditorViewController instance leaks — deinit is never called and Safari's Web Inspector shows lingering web views.

The root cause is EditorService.prepare() storing the progress callback in a property that is never cleared. Since EditorViewController owns the EditorService and the callback captures the view controller, this creates a retain cycle:

EditorViewController → editorService → progressCallback → EditorViewController

This only affects the async flow (no pre-loaded dependencies) because:

• Offline/default editor: prepare() returns early before storing the callback
• Pre-loaded dependencies: The async flow is never entered

In WordPress-iOS, the issue is reproducible by using the pull-to-refresh gesture on My Site to invalidate the editor dependencies cache, then opening the editor.

How?

1. EditorService.swift: Added defer { self.progressCallback = nil; self.progress = nil } in prepare() to clear the stored callback after completion, breaking the cycle from the service side.

2. EditorViewController.swift: Added [weak self] captures in the Task closure and progress callback as defense-in-depth, ensuring the cycle cannot form even if the service retains the callback.

Either fix alone resolves the issue; both together provide robustness.

Testing Instructions

1. Open the iOS demo app
2. Configure a site editor (do not tap "Prepare Editor" to pre-load dependencies)
3. Open the editor — it will load dependencies asynchronously
4. Close the editor
5. Repeat steps 3–4 several times
6. Verify in Safari Web Inspector that old web views no longer linger
7. Optionally add a deinit to EditorViewController and confirm it fires on each dismissal