fix: preserve root runtime attrs for plugin templates

[akroshg]

Summary

• Fix root <template> runtime attrs disappearing from plugin client templates.
• Keep SSR output clean by stripping runtime attrs there.
• Preserve @, :, ?, and FAST f-* attrs only for plugin-facing templates.

Why

PR #307 made the parser preserve authored <template> wrappers. That is correct for SSR, but runtime attrs on the wrapper were stripped before FAST received the client template artifact. Apps with root event handlers, like todo-fast and Edge Downloads BTR, rendered correctly but did not wire those handlers.

This fixes the split at the parser boundary: SSR gets the stripped template, while plugins get the client artifact with root runtime attrs preserved. That is better than changing apps or adding FAST-specific restoration logic.

Validation

• cargo test -p microsoft-webui-parser plugin_artifact_template_keeps_root_runtime_attrs
• cargo test -p microsoft-webui-parser component_template_strips_shadowrootmode_from_f_template
• cargo test -p microsoft-webui-parser fast_v3
• cargo test -p microsoft-webui-parser fast_v2
• pnpm -C examples/app/todo-fast build
• pnpm --dir docs build
• cargo xtask check reaches the existing Windows libc::rusage/getrusage failure in crates/webui/examples/streaming_resource_bench.rs

[akroshg]

#359 has a better fix.

[akroshg]

Closing in favor of #359.