Add meta description, canonical URL, and robots settings for Search Results and 404 pages

### Description

Issue #161's MVP requirements call for the ability to set title/description semi-programmatically for all archive and special pages (post types, taxonomies, authors, homepage, search page, etc). Every other context (Home, post type archives, taxonomy archives, Author archives, Date archives) gets a full settings section via `register_settings_section()` in `WP_SEO_Settings::register_settings()` — title, description, keywords, canonical URL, and robots fields. Search Results and 404 pages only register a title field (`search_title` / `404_title`); no description, canonical URL, or robots settings exist for them.

### Use Case

Site admins want the same level of control over the ``, meta description, canonical URL, and robots directives for search results and 404 pages as they already have for every other archive type, for consistency and to avoid duplicate-content / indexing issues on these pages.

### Proposed Solution

Bring the Search Results and 404 sections up to parity with the other archive-type sections by routing them through `register_settings_section()` (or the equivalent fields it registers) so `search_description`/`404_description`, canonical URL, and robots settings exist and are honored in `wp_title()`/`wp_head()`/`wp_robots()`.

### Related

- #161 (parent MVP tracking issue)
- #169 (Open Graph support for Homepage, Taxonomies, Search and Archive pages) — that issue covers only the Open Graph tags for these contexts, not the base title/description/canonical/robots settings this issue addresses

Start in WP_SEO_Settings::register_settings() and compare the Search Results and 404 registrations with the sections created through register_settings_section(). Then trace wp_title(), wp_head(), and wp_robots() to verify that description, canonical URL, and robots settings are honored for both contexts; done means both sections provide and use those settings consistently.

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