feat: export PostUpdateParams in Swift public API (#1270)

* feat: export PostUpdateParams in Swift public API

PostUpdateParams was already defined in Rust and available via uniffi
bindings in WordPressAPIInternal, but was not re-exported in the
public WordPressAPI module's Exports.swift. This prevents consumers
like GutenbergKit from constructing params to call
posts.updateCancellation() without also importing WordPressAPIInternal.

Also adds a compatibility test following the existing pattern in
native/swift/Tests/api-compatibility/ that validates the type is
constructible from the public module.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: Remove rudimentary PostsCompatTests

This test covers very little functionality directly. While there are
other existing tests like it, we decided it is not worth adding.

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dcalhoun

native/swift/Sources/wordpress-api/Exports.swift

@@ -134,6 +134,7 @@ public typealias AnyPostWithEditContext = WordPressAPIInternal.AnyPostWithEditCo
 public typealias AnyPostWithViewContext = WordPressAPIInternal.AnyPostWithViewContext
 public typealias AnyPostWithEmbedContext = WordPressAPIInternal.AnyPostWithEmbedContext
 public typealias PostListParams = WordPressAPIInternal.PostListParams
+public typealias PostUpdateParams = WordPressAPIInternal.PostUpdateParams
 public typealias PostsRequestExecutor = WordPressAPIInternal.PostsRequestExecutor
 public typealias PostEndpointType = WordPressAPIInternal.PostEndpointType