perf: fix O(N^2) accumulation in MessageStream and BetaMessageStream

[claygeo]

Summary

Replaces object spread with direct in-place mutation in #accumulateMessage for all content_block_delta types in both MessageStream and BetaMessageStream. The previous implementation created a new content block object via { ...snapshotContent, prop: newValue } on every delta event, causing O(N^2) work for N deltas.

Delta types fixed:

• text_delta: { ...obj, text: old + new } → obj.text = old + new
• citations_delta: [...existing, new] → Array.push() (O(N^2) → O(N) for array copy)
• input_json_delta: spread + defineProperty each time → defineProperty once, then direct assignment
• thinking_delta: { ...obj, thinking: old + new } → obj.thinking = old + new
• signature_delta: { ...obj, signature: val } → obj.signature = val
• compaction_delta (Beta only): { ...obj, content: old + new } → obj.content = old + new

Why this is safe: The message_delta case already uses direct mutation (snapshot.stop_reason = event.delta.stop_reason). All content block TypeScript types are mutable interfaces (no readonly modifiers). The snapshot is stored in a private field and events emit references to the same object.

Also fixes: BetaMessageStream's content_block_start was missing the defensive copy ({ ...event.content_block }) that MessageStream has. Added for consistency since we now mutate content blocks in place.

Fixes #995
Fixes #933

Relationship to PR #936

PR #936 addresses only input_json_delta with a lazy getter approach. This PR fixes all 6 delta types in both files using a simpler, consistent mutation pattern.

Test plan

• All existing tests pass (MessageStream.test.ts, BetaMessageStream.test.ts, streaming.test.ts)
• Added test: "accumulates large text responses in linear time" — streams 5,000 text deltas and verifies correct accumulation
• tsc --noEmit passes with no type errors