Prototype: replace WpErrorCode fallback with WpErrorCodeValue record

[oguzkocer]

Description

Replace the CustomError(String) fallback variant in WpErrorCode with a new WpErrorCodeValue record that separates the parsed enum from the raw API string:

• value: Option<WpErrorCode> — Some for known variants, None for unknown
• raw: String — always the original string from the API

This makes adding new WpErrorCode variants forward-compatible: client code checking the raw string continues to work after a variant is promoted, and the enum stays a real enum with full match support.

Also changes WpErrorCode from uniffi::Error to uniffi::Enum since it was never returned as an error directly — always embedded as a field.

Helper methods on WpErrorCodeValue: is_code(), is_raw(), is_any_code().

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]

⚠️ CHANGELOG.md was not updated in this PR.

Every PR should add an entry under the ## [Unreleased] section of CHANGELOG.md describing the change for our users. The format follows Keep a Changelog 1.0.0 — use one of:

• ### Added — for new features
• ### Changed — for changes in existing functionality (prefix **BREAKING:** if breaking)
• ### Deprecated — for soon-to-be-removed features
• ### Removed — for now-removed features
• ### Fixed — for any bug fixes
• ### Security — for vulnerability fixes

If the change genuinely has no user-visible impact (e.g. CI-only tweaks, internal refactors), add a short entry under ### Changed noting that.

[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/1365")

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