Agent Runtime: Empty {"content": "", "role": "assistant"} record appears on session resume

[wangtsiao]

Describe the bug

When exiting the app and resuming a session, an empty assistant message {"content": "", "role": "assistant"} appears in the conversation history.

Current behavior

On session load/resume, a spurious record with empty content and assistant role shows up. This appears to be a ghost record that gets written during session teardown and then re-read on resume.

Impact

• Confusing for users seeing an empty assistant message
• May affect context window usage (waste tokens on empty messages)
• Could be a symptom of improper session serialization/deserialization

Where to look

• Session serialization (save/load) path
• crates/core/src/session.rs — session state
• crates/core/src/session_store.rs — session persistence
• Rollout file reading on resume

Expected behavior

No empty assistant messages should appear in the conversation. Session serialization should filter out empty or orphaned records.