Add dedicated clients for all missing WordPress REST API endpoints

[Copilot]

Implements the standard wp/v2 endpoints listed as gaps in docs/v3/endpoint-coverage.md. Adds 11 new top-level clients, 3 sub-resource helpers, 11 models, and 6 query builders.

New top-level clients on WordPressClient

Property	Endpoint	Operations
client.Search	wp/v2/search	query by term or SearchQueryBuilder
client.BlockTypes	wp/v2/block-types	read, GetByNameAsync("core","paragraph")
client.Blocks	wp/v2/blocks	full CRUD (reusable blocks)
client.Templates	wp/v2/templates	full CRUD, string compound IDs
client.TemplateParts	wp/v2/template-parts	full CRUD, string compound IDs
client.GlobalStyles	wp/v2/global-styles/{id}	GetByIdAsync, GetThemeStylesAsync, UpdateAsync
client.Navigation	wp/v2/navigation	full CRUD
client.Sidebars	wp/v2/sidebars	read + update (string IDs)
client.Widgets	wp/v2/widgets	full CRUD + GetBySidebarAsync (string IDs)
client.WidgetTypes	wp/v2/widget-types	read (string IDs)
client.UrlDetails	wp/v2/url-details	GetAsync(url) — requires auth

New sub-resource helpers

// Post autosaves (new)
var autosaves = await client.Posts.Autosaves(postId).GetAsync();

// Page revisions and autosaves (both new)
var revisions = await client.Pages.Revisions(pageId).GetAsync();
var autosaves  = await client.Pages.Autosaves(pageId).GetAsync();

Design notes

• Templates, TemplateParts, Sidebars, Widgets, WidgetType all use string IDs (e.g. "twentytwentyfour//index", "sidebar-1"); they have custom CRUD implementations instead of CRUDOperation<T,Q> which assumes integer IDs.
• Blocks and Navigation extend Base (integer ID) and reuse CRUDOperation<T,Q> normally.
• GlobalStylesClient and UrlDetailsClient are named with a Client suffix to avoid clashing with the identically-named model classes.

New models

SearchResult, BlockType, Block, Template, TemplatePart, GlobalStyles, Navigation, Sidebar, Widget, WidgetType, UrlDetails (+OgImage)

New query builders

SearchQueryBuilder, BlocksQueryBuilder, TemplatesQueryBuilder, TemplatePartsQueryBuilder, NavigationQueryBuilder, WidgetsQueryBuilder

Docs + tests

• docs/v3/endpoint-coverage.md and README.md updated to reflect the new coverage table and remaining gaps.
• Self-hosted integration tests added for all new clients.