fix: trap when a mediaUploader is set without site credentials

[jkmassel]

Stacked on #631. Ninth of ten PRs splitting #621.

What?

Setting a mediaUploader without site credentials is a configuration error, and now fails fast instead of silently doing nothing.

Why?

Setting a mediaUploader means the host is taking over uploads. With no site credentials the server would previously just not start, silently dropping the uploader — and its media deletes still need the internal media client to reach the configured site, since every attachment lives there no matter who delivered it. Starting anyway would give a server whose every delete 500s.

How?

The behavior forks by intent:

• A mediaProcessor with no credentials leaves the server down and uploads fall to the default WebView path — there is nothing to deliver through, so nothing to process.
• A mediaUploader with no credentials traps: precondition on iOS, check on Android.

The iOS policy lives in MediaServerCredentials (added in #624) rather than EditorViewController, which is #if canImport(UIKit) and therefore absent from the macOS host — the one platform that can run Swift Testing's exit tests. Living outside the gate, the trap itself is testable, not just the predicate.

Testing Instructions

• Two iOS exit tests run the trap in a child process; neutering the precondition fails both
• Three Android tests through GutenbergView — the two uploader arms and the processor arm; neutering the check fails the two uploader tests
• swift test — host suite green
• Android :Gutenberg:testDebugUnitTest green
• iOS Simulator xcodebuild
• SwiftLint + Detekt 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/632")

_{Built from 9ee12ea}