fix(ios): own the media upload delegate instead of holding it weakly

[jkmassel]

Stacked on #624. Second of ten PRs splitting #621.

What?

A host that releases its mediaUploadDelegate while an upload is in flight gets the file forwarded to WordPress unprocessed. Hold the delegate strongly, as Android already does.

Why?

The server reads the delegate three times per request — once at the admission gate (handlesFile), then again for processFile and uploadFile. Those reads are separated by a synchronous disk copy and an unbounded processFile. Held weakly, a host releasing its delegate in that window changes the answer between them: a file admitted for processing is delivered untouched.

The weak bought no leak protection to trade away. The cycle it named runs through EditorViewController.mediaUploadDelegate — a host object retaining the view controller forms EditorViewController → delegate → EditorViewController regardless of how the server's container holds it. What it did buy was the reference vanishing mid-request.

How?

• MediaUploadServer.swift: UploadContext holds the delegate strongly and becomes a struct, dropping its @unchecked Sendable opt-out — MediaUploadDelegate is Sendable and DefaultMediaUploader is @unchecked Sendable, so it is implicitly Sendable. Immutable strong references make the three reads agree by construction.
• EditorViewController.swift: mediaUploadDelegate becomes strong. The machinery that existed only to police the old contract goes with it — mediaUploadDelegateWasAssigned and the released-before-load trap have nothing left to catch. Hosts no longer need to retain the delegate themselves.

SwiftLint's weak_delegate is suppressed with the reasoning inline. The rule is arguably right that the name no longer fits; #630 renames the property and the suppression goes away with it.

Testing Instructions

doesNotStronglyRetainDelegate pinned the invariant being removed, so it is replaced by retainsDelegateForServerLifetime, asserting both halves — the server owns the delegate while it runs, and releases it afterward. processesForHostReleasedDelegate covers the bug directly.

• Both fail against a weak container, with the real symptom: passthroughUploadCalled → true
• swift test — host suite green
• iOS Simulator xcodebuild
• SwiftLint clean

[wpmobilebot]

XCFramework Build

This PR's XCFramework is available for testing. Add the following to your Package.swift:

.package(url: "https://github.com/wordpress-mobile/GutenbergKit", branch: "pr-build/625")

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