docs: update stream-collapse module comment to reflect content+toolCalls coexistence

Author: AlemTuzlak

src/stream-collapse.ts

@@ -3,7 +3,8 @@
  *
  * Each function takes a raw streaming response body (SSE, NDJSON, or binary
  * EventStream) and collapses it into a non-streaming fixture response
- * containing either `{ content }` or `{ toolCalls }`.
+ * containing `{ content }`, `{ toolCalls }`, or both when the stream includes
+ * text followed by tool calls.
  */
 
 import { crc32 } from "node:zlib";