fix: shallow copy content_block in BetaMessageStream#accumulateMessage

[MaxwellCalkin]

Summary

• Fix missing defensive copy in BetaMessageStream: The content_block_start case in BetaMessageStream#accumulateMessage pushes event.content_block directly into the snapshot's content array, while the non-beta MessageStream correctly uses { ...event.content_block } to create a shallow copy.

Details

In src/lib/MessageStream.ts (line 600):

case 'content_block_start':
    snapshot.content.push({ ...event.content_block }); // ✅ shallow copy
    return snapshot;

In src/lib/BetaMessageStream.ts (line 606, before this fix):

case 'content_block_start':
    snapshot.content.push(event.content_block); // ❌ direct reference
    return snapshot;

Without the copy, the snapshot holds a direct reference to the event object. Any external code that retains and mutates the original event's content_block would inadvertently corrupt the accumulated message snapshot. The rest of the content_block_delta handling in both files already creates new objects via spread ({ ...snapshotContent, ... }), so this is specifically a gap in the content_block_start case of the beta stream.

This is a one-character fix ({ ...event.content_block } instead of event.content_block) that aligns BetaMessageStream with the existing MessageStream behavior.

Test plan

• Verify existing tests pass
• The fix is a minimal change that matches the pattern already used in MessageStream
• All subsequent content_block_delta handlers already create new objects via spread, so this only affects the initial content block reference

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