The openUrl function in @a2ui/web_core passes an agent-controlled URL directly to window.open() without validating the URI scheme. A malicious agent can supply a javascript: URI as the url argument of a Button component's functionCall action. When the user clicks the rendered button, arbitrary JavaScript executes in the victim application's browser origin, constituting a stored/reflected XSS with Critical severity. No non-default configuration is required; the Basic Catalog is enabled by default.

Affected: npm: @a2ui/web_core

### Summary

The `openUrl` function in `@a2ui/web_core` passes an agent-controlled URL directly to `window.open()` without validating the URI scheme. A malicious agent can supply a `javascript:` URI as the `url` argument of a `Button` component's `functionCall` action. When the user clicks the rendered button, arbitrary JavaScript executes in the victim application's browser origin, constituting a stored/reflected XSS. No non-default configuration is required; the Basic Catalog is enabled by default.

### Details

The vulnerability exists in the `openUrl` function implementation within the Basic Catalog of `@a2ui/web_core` (commit `23a003248abbf59da6c376ea64ace91d82d209ff`).

**Sink** — `renderers/web_core/src/v0_9/basic_catalog/functions/basic_functions.ts:428-430`:

```ts export const OpenUrlImplementation = createFunctionImplementation(OpenUrlApi, args => { if (args.url && typeof window !== 'undefined' && window.open) { window.open(args.url, '_blank'); } }); ```

`window.open` is called unconditionally with the agent-supplied `args.url` value. No scheme allowlist or blocklist is applied.

**Insufficient schema validation** — `renderers/web_core/src/v0_9/basic_catalog/functions/basic_functions_api.ts:453-458`:

```ts export const OpenUrlApi = { name: 'openUrl' as const, returnType: 'void' as const, schema: z.object({ url: z.preprocess(v => (v === undefined ? undefined : String(v)), z.string()), }), }; ```

The Zod schema only requires a `string`; `javas…