react-google-adk: an event-stream response without a body throws a TypeError

Problem

A successful ADK response with the expected text/event-stream content type but no response body throws an internal Cannot read properties of null TypeError.

Reproduction

Return new Response(null, { status: 200, headers: { "Content-Type": "text/event-stream" } }) from the configured fetch call and start consuming the ADK stream.

Root cause

The response content type is validated, but parseSSEResponse uses response.body!.getReader() without validating that the body exists.

Proposed fix

Validate the response body before creating the reader and throw a package-level ADK response error that explains the missing stream body.

Verification

Add a focused client test asserting the stable error message for a successful event-stream response with a null body.