fix(plugin-rsc): expose encodeFormAction option type and re-export encodeReply from ssr

[dai-shi]

Two gaps hit when a framework customizes React's progressive-enhancement
form encoding during SSR. The first is required for wakujs/waku's pending
change; the second is a type-only convenience.

1. Missing export (blocking). A custom encodeFormAction that reproduces
   React's default $ACTION_REF_ field encoding needs encodeReply, which
   the vendored client.edge exports but the /ssr (and /react/ssr)
   wrappers do not re-export. Importing it from /ssr, the same entry that
   provides createFromReadableStream, guarantees the serializer matches
   the flight-client instance in use regardless of which react-server-dom
   build or version is active, and avoids downstream deep imports into
   vendor/. One-line re-export:

   // react/ssr.ts and ssr.ts
   export { encodeReply } from './vendor-or-resolved-client-edge';

2. Type gap. createFromReadableStream from @vitejs/plugin-rsc/ssr
   spreads ...options into the vendored flight client, so
   encodeFormAction works at runtime today, but the published options type
   (CreateFromReadableStreamEdgeOptions) omits it, forcing a cast:

   export type CreateFromReadableStreamEdgeOptions = {
     nonce?: string;
     // add:
     encodeFormAction?: (
       actionId: string,
       boundPromise: Promise<unknown[]>,
     ) => ReactCustomFormAction;
   };

[dai-shi]

Can anyone trigger preview please?