feat!: remove MediaUploadDelegate.uploadFile

[jkmassel]

Stacked on #628. Sixth of ten PRs splitting #621. Almost entirely deletions.

What?

Removes uploadFile from MediaUploadDelegate on both platforms. MediaUploader (#628) replaces it.

Why?

Returning a raw response split one upload's HTTP across two owners — the host performed the POST, the editor drove the post-process retries and orphan cleanup behind it — and the hook received no form fields, so an attachment it uploaded landed unattached to its post. Neither is fixable while the hook returns a raw response.

What is left is a clean division: a delegate transforms bytes and GutenbergKit owns delivery and its retries; a MediaUploader owns delivery and its retries entirely. There is no longer an in-between where the host performs the upload but the editor retries it.

How?

• The protocol requirement, its default implementation, and the delivery branch that called it, on both platforms.
• handlesFile no longer gates the temp copy for two callers, only for processFile. With a MediaUploader set it can't decline the upload itself — an uploader delivers every file, so there is no passthrough to fall to — but it still gates processFile, so a declined file reaches the uploader unprocessed. Pinned by an uploader sees a file the delegate's metadata gate would have declined on both platforms.
• MediaUploadResponse drops to internal on both platforms: uploadFile was the only public API that named it. This is load-bearing for migration safety — Swift requires an explicit return type, so a host's stale uploadFile cannot compile without naming the type. Leave it public and the same removal breaks hosts silently instead.
• The deprecation warning feat: add MediaUploader, for a host that owns the whole upload #628 left in GutenbergKit's own build goes away with the hook.

A second commit corrects the docs the removal left stale: the mediaUploadDelegate property summaries — what Xcode Quick Help and IDE hover show — still advertised "custom upload" on both platforms and pointed nowhere at MediaUploader, and UploadError.noUploader's message still named the delegate as an alternative to the internal client.

Testing Instructions

Tests for the removed path are rewritten rather than deleted — the delegate now processes and the internal client delivers, which is what the remaining path does.

• swift test — 974 tests in 57 suites, green
• xcodebuild test -scheme GutenbergKit-Package -sdk iphonesimulator — 585 tests in 36 suites, green. Covers the UIKit-gated code the host swift test compiles as empty, including EditorViewController.
• :Gutenberg:testDebugUnitTest — 672 tests, 0 failures
• :app:assembleDebug — Android demo builds
• swift build --build-tests — zero warnings
• SwiftLint and Detekt clean

Breaking change

Hosts implementing uploadFile must conform to MediaUploader instead. Hosts that only implement processFile / handlesFile are unaffected. Migration needed in WordPress-iOS, WordPress-Android and Jetpack.

The break is a hard compile error on both platforms, not a silent redirect — Android on override of nothing, iOS on MediaUploadResponse no longer being visible. uploadFile shipped in v0.20.0-alpha.0 and no stable tag.

[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/629")

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