fix(streaming): retain encrypted_content metadata when omitted from compaction_delta (#1984)

[iosayin]

Summary

Retains previously accumulated encrypted_content metadata on a compaction block when a subsequent compaction_delta does not include the encrypted_content field.

Previously, accumulate_event unconditionally assigned content.encrypted_content = event.delta.encrypted_content for every compaction_delta. Because BetaCompactionContentBlockDelta.encrypted_content defaults to None when omitted from the payload, a content-only update (e.g. {"type": "compaction_delta", "content": "Updated summary."}) cleared the previously accumulated metadata.

With this change, encrypted_content is only updated when explicitly provided in the delta payload ("encrypted_content" in model_fields_set(event.delta)), preserving previous metadata on content-only updates while still allowing explicit nulls to clear it.

Closes #1984