Return a ResolvedUrl from resolve() with a canonical preload route key

[jkmassel]

Description

Evolves the REST URL resolver so ApiUrlResolver.resolve returns a ResolvedUrl instead of a bare ParsedUrl. ResolvedUrl carries both halves a preloading editor needs: the request URL to fetch (url()) and the canonical, origin-less route key that @wordpress/api-fetch's preload middleware matches on (canonical_route_key()). This lets wprs own the preload cache key natively instead of asking GutenbergKit to re-implement api-fetch's normalizePath.

Stacked on #1549 (issue-1543), which ships ParsedUrl.by_appending_query_pairs — the primitive ResolvedUrl builds on. This targets issue-1543, so #1549 becomes the ResolvedUrl PR.

Why canonical_route_key

api-fetch matches an outgoing request to a preload key by running both through normalizePath, which is origin-less, keys on path + query, sorts query params on both sides, unwraps the ?rest_route= form back to the canonical /wp/v2/… path, and early-returns when there's no query. So the key GBK precomputes natively is the canonical /wp/v2/themes?context=edit&status=active — not the full request URL (origin isn't stripped) and not a query-less path (won't match a query-bearing request). by_appending_query_pairs gave GBK the request URL but nothing for the key; route_path gave a query-less path. ResolvedUrl closes that gap.

Changes

• wp_api/src/resolved_url.rs: new ResolvedUrl (uniffi::Object) with url() (the request URL as a string, mirroring ParsedUrl.url()), parsed_url() (the same as a ParsedUrl object — the escape hatch), by_appending_query_pairs(), and canonical_route_key(), plus an exported constructor so foreign ApiUrlResolver impls can build one. It carries the request URL, the API root it was resolved against, and a memoized canonical route path — so the key is a pure concat, guaranteed to match route_path().
• ApiUrlResolver::resolve now returns Arc<ResolvedUrl> across the trait and all three resolvers (WpOrgSiteApiUrlResolver, WpComDotOrgApiUrlResolver, WpComApiClientInternalUrlResolver), each stamping the value with the root it used and its own route_path.
• The endpoint-builder macro funnel (wp_derive_request_builder) and the other internal call sites unwrap to the request URL with .url(), so request-building is byte-identical.
• canonical_route_key(): origin-less path[?query]; unwraps ?rest_route= back to /wp/v2/…; drops rest_route; omits the ? when there is no query. Invariant: byte-identical on pretty (…/wp-json/…) and plain (…?rest_route=…) permalink sites.
• Swift: public typealias ResolvedUrl in Exports.swift (new UniFFI objects are invisible to Swift consumers without it; only swift test catches the omission). Kotlin needs no re-export.
• CHANGELOG: BREAKING (resolve signature) + Added (canonical_route_key).

BREAKING

ApiUrlResolver.resolve now returns ResolvedUrl instead of ParsedUrl. This is a with_foreign trait change — the accepted trade-off for wprs owning the cache key. resolve(...).url() still returns the request URL as a string, so that call pattern is unchanged; the return type is what differs. Apps with a custom ApiUrlResolver must return a ResolvedUrl (build one with ResolvedUrl(url:apiRoot:routePath:)), and code that used the resolved value as a ParsedUrl object reads resolve(...).parsed_url().

Test plan

• cargo test -p wp_api --lib — 1967 pass, including new ResolvedUrl unit tests and a resolver round-trip proving the pretty/plain key invariant (no-query, multi-param, encoded value, embedded-slash route)
• plain_permalinks_url_tests (45) and index_self_href_url_tests (30) pass unchanged — the funnel change did not alter request-building
• New integration test asserts a resolved plain-permalinks endpoint's canonical_route_key() equals its pretty-permalinks counterpart across real route shapes
• cargo clippy --tests --all-targets --all-features -- -D warnings clean; cargo fmt --all -- --check clean
• make xcframework-only-macos && swift build --target WordPressAPI — Exports.swift compiles against the regenerated bindings
• BUILDKITE=1 swift test — 82 tests in 18 suites pass
• xcrun swift format lint --strict clean
• cd native/kotlin && ./gradlew :api:kotlin:compileKotlin :api:kotlin:detekt — BUILD SUCCESSFUL

Changelog

• I've added an entry to CHANGELOG.md under ## [Unreleased], using the Keep a Changelog categories (Added, Changed, Deprecated, Removed, Fixed, Security). Prefix breaking changes with **BREAKING:**.

[wpmobilebot]

XCFramework Build

This PR's XCFramework is available for testing. Add to your Package.swift:

.package(url: "https://github.com/automattic/wordpress-rs", branch: "pr-build/1599")

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