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 — and only when no uploader is set.
• MediaUploadResponse drops to internal on both platforms: uploadFile was the only public API that named it.
• 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.

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 — host suite green
• Android :Gutenberg:testDebugUnitTest green
• iOS Simulator xcodebuild; Android demo compiles
• SwiftLint + 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.

[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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