feat: re-export hydration plugins from microsoft-webui facade

[janechu]

Summary

Adds pub use re-exports for FastV3HydrationPlugin and WebUIHydrationPlugin from the microsoft-webui facade crate.

Problem

microsoft-webui already re-exports WebUIHandler, HandlerError, ResponseWriter, and the HandlerPlugin trait from microsoft-webui-handler. But the concrete hydration plugin types - FastV3HydrationPlugin and WebUIHydrationPlugin - are not re-exported. Every embedder that wants client-side hydration must add microsoft-webui-handler as a direct dependency just to write one line like:

use webui_handler::plugin::fast_v3::FastV3HydrationPlugin;

even when no other handler-only API is used. The facade is otherwise complete, so this is a paper cut that doubles the dependency surface for integrations.

Change

Two lines in crates/webui/src/lib.rs:

pub use webui_handler::plugin::fast_v3::FastV3HydrationPlugin;
pub use webui_handler::plugin::webui::WebUIHydrationPlugin;

After this change, a typical embedder needs only microsoft-webui as a direct dep:

use webui::{FastV3HydrationPlugin, RenderOptions, ResponseWriter, WebUIHandler, WebUIProtocol};

let handler = WebUIHandler::with_plugin(|| Box::new(FastV3HydrationPlugin::new()));

The deep path webui_handler::plugin::fast_v3::FastV3HydrationPlugin remains available for embedders that depend on microsoft-webui-handler directly, so this is purely additive.

Why this pair

These two plugins are the only concrete HandlerPlugin impls shipped today. The pair matches the existing facade pattern where (CssStrategy, DomStrategy, Plugin) parser-side types are re-exported alongside the parser Plugin trait - all the wiring types an embedder needs to construct a render pipeline now live on webui.

Verification

• cargo check -p microsoft-webui passes (clean compile)
• No existing import paths break (additive only)
• Tested by an embedded integration that consumes microsoft-webui only and constructs both plugins via the facade

Companion PRs

• chore: correct rust integration docs to match published crate #295 (still open): re-exports RenderOptions and adds a HandlerResult alias on the same facade. Together with this PR, an integration can drop microsoft-webui-handler from Cargo.toml entirely.
• docs: add embedded fragment rendering recipe page #299 (still open): documents single-fragment embedded rendering, which is the primary integration shape that benefits from the facade simplification.