fix: add defensive copy in BetaMessageStream content_block_start

Add object spread when pushing content blocks in BetaMessageStream to
match MessageStream behavior. Since we now mutate content blocks in
place during delta accumulation, this prevents the original event
object from being modified.

Author: claygeo

src/lib/BetaMessageStream.ts

@@ -603,7 +603,7 @@ export class BetaMessageStream<ParsedT = null> implements AsyncIterable<BetaMess
 
         return snapshot;
       case 'content_block_start':
-        snapshot.content.push(event.content_block);
+        snapshot.content.push({ ...event.content_block });
         return snapshot;
       case 'content_block_delta': {
         const snapshotContent = snapshot.content.at(event.index);