.NET: [Durable Agents] Filter empty AIContent from durable agent state responses

[cgillum]

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 #4481, which has more context

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Updates DurableTask state serialization to avoid persisting response messages that contain only non-serializable/opaque AIContent, reducing noisy/empty durable state entries.

Changes:

• Filter AgentResponse.Messages during DurableAgentStateResponse.FromResponse to keep only messages with serializable content.
• Add unit tests covering the filtering behavior (drop opaque-only messages; keep mixed content; keep base AIContent when it has annotations/additional properties).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.DurableTask/State/DurableAgentStateResponse.cs	Filters out response messages that would serialize as empty/meaningless state.
dotnet/tests/Microsoft.Agents.AI.DurableTask.UnitTests/State/DurableAgentStateResponseTests.cs	Adds test coverage for the new filtering rules.

---

You can also share your feedback on Copilot code review. Take the survey.