feat(ios): add HTTPRequestHandler, and serve media uploads from one

[jkmassel]

Stacked on #685. Seventh of nine PRs splitting #621. No behavior change — the review is "did any body change?"

What?

One commit, two parts:

1. HTTPRequestHandler in GutenbergKitHTTP

An HTTPServer.start overload taking a handler object rather than a closure. Purely additive: the closure overload is unchanged and forwards to the same code path, so no existing caller, test, or the debug server is affected.

2. MediaUploadServer serves from one

The statics threading an UploadContext become a Handler struct with stored properties.

Why?

The closure form of start can't capture the object that owns the server: the closure has to exist before the server does, and retrofitting self would form MediaUploadServer → HTTPServer → handler → MediaUploadServer, so the owner's deinit — and its stop() — would never run. A consumer with dependencies to hold therefore ends up with static functions threading a context parameter through every call, which is how MediaUploadServer is written today.

HTTPRequestHandler is deliberately not AnyObject-constrained so a handler can be a struct holding only what it needs — not because a struct is safe by construction. A value type is not protection: the server captures the handler into a heap node, so a struct storing the server's owner closes owner → HTTPServer → handler → owner exactly as a class would. Both shapes work, under the same leaf discipline HTTPServerDelegate already documents — a handler must not strongly hold the object that owns the server. Handler stores no reference back to the MediaUploadServer, which is why the helpers outside it stay static.

Request handling is mandatory, so it can't be a defaulted HTTPServerDelegate method the way optional customization points are — hence an overload rather than a new delegate requirement.

How?

handleRequest becomes handle; the functions that use the dependencies become instance methods, and the ones that don't (attachmentId, relayResponse, uploadErrorResponse, formFields) stay static. UploadContext goes away — Handler is what it was. Helpers outside the handler (errorResponse, writeStream, sanitizeFilename, uploadsTempDirectory) are qualified rather than moved.

The restructure is confined to one source file — MediaUploadServer.swift, 296/299 lines of it pure churn — plus a one-line comment in its test naming the retain loop. Read that file as "did any body change?"; the API design is the other four files.

Testing Instructions

• swift test — host suite green, unchanged
• One new test for the HTTPServer overload
• 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/686")

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