reconstructFromMessages

Reconstruct subagent state from historical messages.

This method parses an array of messages (typically from thread history) to identify subagent executions and their results. It's used to restore subagent state after:

• Page refresh (when stream has already completed)
• Loading thread history
• Navigating between threads

The reconstruction process:

1. Find AI messages with tool calls matching subagent tool names
2. Find corresponding tool messages with results
3. Create SubagentStream entries with "complete" status

Note: Internal subagent messages (their streaming conversation) are not reconstructed since they are not persisted in the main thread state.