* Filter empty AIContent from durable agent state responses
Prevent opaque AIContent objects (e.g., with only RawRepresentation set)
from being stored in durable entity state, where they serialize to empty
JSON payloads. Base AIContent instances are kept only if they have
Annotations or AdditionalProperties.
Fixes https://github.com/microsoft/agent-framework/issues/4481
* Update CHANGELOG.md and fix linter violation
Chris Gillum