[Lushiyao5]

What

The beta streaming accumulator assigned encrypted_content from every compaction_delta. The generated delta model defaults an absent field to None, so a later content-only delta ({"type":"compaction_delta","content":"Updated summary."}) erased metadata received earlier in the same block. The accumulator now updates encrypted_content only when the field was actually present in the delta.

Why

encrypted_content is opaque metadata from a prior compaction that must be round-tripped verbatim. Treating "field omitted" as "clear the field" makes the value depend on the order of deltas: any content-only update silently drops it. A field that is explicitly sent must still take effect, including null (clear) and "" (replace with empty).

How

elif event.delta.type == "compaction_delta":
    if content.type == "compaction":
        content.content = event.delta.content
        if "encrypted_content" in event.delta.model_fields_set:
            content.encrypted_content = event.delta.encrypted_content

model_fields_set distinguishes an omitted field from an explicit null. Summary text accumulation is unchanged.

Tests

Added a parameterized test in tests/lib/streaming/test_parsed_content_blocks.py driving accumulate_event with a populated compaction block followed by a content-only delta, for four cases:

delta encrypted_content	expected
omitted	retains "opaque"
null	None
""	""
"new"	"new"

• Before: → 1 failed, 8 passed (only the omitted case fails).
• After: → 9 passed.
• ruff check and ruff format --check pass.

Fixes #1984