refactor: deduplicate session-state branch handling

[LIU9293]

Summary

• extract shared assistant/user/result branch handling into applyAssistantUserResultBranches in session-state shared utilities
• switch Claude, Qwen, Goose, and Kilo session-state adapters to the shared branch handler while preserving provider-specific pre-processing
• remove duplicated helper implementations by reusing shared updateTool, parseTodosFromToolInput, and extractPrefixedRecord

Testing

• bun test packages/agents/test