fix(core): simplify ReActAgent pending tool and error result handling (#2666)

## Summary

- Simplify `ReActAgent.summarizing()` by directly checking the extracted
pending tool calls.
- Move tool execution error-result construction into `ToolResultBlock`.
- Remove the unused `stateUpdates` map from `applyConfirmResults`.
- Preserve the existing error state and `[ERROR]` marker behavior.

## Checklist

Please check the following items before code is ready to be reviewed.

- [ ]  Code has been formatted with `mvn spotless:apply`
- [ ]  All tests are passing (`mvn test`)
- [ ]  Javadoc comments are complete and follow project conventions
- [ ] Related documentation has been updated (e.g. links, examples,
etc.)
- [ ]  Code is ready for review