refactor: rename DefaultMediaUploader to InternalMediaClient

[jkmassel]

Stacked on #625. Third of nine PRs splitting #621. No behavior change beyond error-message text.

What?

DefaultMediaUploader → InternalMediaClient on both platforms, and the defaultUploader parameters and properties that carry it.

Why?

The name reads as an implementation of a host-facing protocol — the "default" one, as against a host's. It is not. It is GutenbergKit's own HTTP client for the configured site: it performs the uploads no host took over, and it relays every media delete, because the editor only ever asks to delete /wp/v2/media/<id> on the configured site.

The name gets actively confusing in #683, which adds a MediaUploader protocol. Renaming first keeps that PR's diff about the new API rather than about disambiguating two similar names.

How?

Mechanical, ~21 sites on trunk plus tests, plus a sweep of the prose and error strings that used the retired vocabulary — including the UploadContext doc header and Android's three media-client messages. The host-facing docs still say "the default uploader" as a role: InternalMediaClient is internal on both platforms, so naming it in prose a host reads would be worse.

On iOS it also narrows two signatures: passthroughResponse and handleDelete took the whole UploadContext and touched only the client, so they now take it directly.

On the delete path that is more than tidiness — a deletion always relays to the configured site, never to a delegate. That was a convention the signature let you break; now the type won't. The three functions that keep the context genuinely need every field.

Android's server holds the client as a constructor property rather than threading a context, so it needs the rename only — and because its handleDelete is an instance method with the delegate in scope, the delete-path convention stays a convention there. The type-level guarantee is iOS-only.

Testing Instructions

• swift test — host suite green (969 tests, 57 suites)
• 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/682")

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