Add a parse test for the minimal private-site API root

[jkmassel]

Description

Adds a validation test that the minimal REST API root a private site advertises parses into WpApiDetails. A private site serves a stripped-down /wp-json/ index — just enough for a client to discover the application-password login endpoint — and this locks in that the library reads it correctly.

No library behavior change: WpApiDetails already parses this shape. The test guards that contract against regressions.

Changes

• test-data/api-details/test-case-08.json (new): the minimal private-site root — application-passwords authentication only, empty namespaces and routes ({}), gmt_offset: 0, and no timezone_string/site_icon_url.
• wp_api/src/login.rs: test_parse_private_site_api_root asserts the fixture parses and that find_application_passwords_authentication_url() returns the authorize endpoint. The fixture is also added to the test_api_details_json parse list, with a pointer comment to the new README.
• test-data/api-details/README.md (new): a catalogue of all eight api-details fixtures — what each exercises, plus the recurring WordPress serialization quirks they cover ([] vs {} empty maps, gmt_offset as string-or-number, site_icon_url as false/string/absent, and the UTF-8 BOM). Retroactively documents the previously-undocumented cases 01–07.
• CHANGELOG.md: an **Internal:** entry under ### Changed.

Notes

The fixture uses "routes": {}, matching what a correct emitter produces. The parser is intentionally left strict — routes is a plain HashMap, so a routes: [] (PHP's json_encode([]) for an empty map) would fail this test rather than be silently tolerated. That is the desired behavior: a malformed root surfaces as a loud failure. Empty authentication is the one field that already tolerates [], because WordPress core itself emits it that way (see test-case-04.json).

Test plan

• cargo test -p wp_api --lib -- login::tests — all parse tests pass, including test_parse_private_site_api_root and the new test_api_details_json case.
• cargo fmt --all -- --check — clean.
• cargo clippy -p wp_api --tests --all-features -- -D warnings — clean.

Changelog

• I've added an entry to CHANGELOG.md under ## [Unreleased], using the Keep a Changelog categories — an **Internal:** note under ### Changed (no user-facing behavior change).

[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/1606")

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