Feature/delta team/epic r04 (#7)

## Summary

epic r04 - begin refactor

## Change Type

- [x] Cowork feature
- [ ] Bug fix
- [ ] Core AI contribution
- [ ] Test / hardening
- [ ] Performance
- [ ] Documentation

## Related Work

Cowork Task:

Core Repo: http://34.143.229.138/gitea-admin/fsg-ai-core-assets

Core AI Issue:

Core Task:

Related PR:

## Scope

What is intentionally included?

What is intentionally NOT included?

## Validation

- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual verification
- [ ] Regression check

Commands / evidence:

## Security Impact

Permission / credential / network / customer data impact:

## Compatibility

- [ ] No breaking change
- [ ] Breaking change documented

## Reviewer Notes

Anything Cowork reviewers should pay attention to.

---------

Co-authored-by: Anh Tran Nguyen Minh <anhtnm1@fpt.com>
Co-authored-by: Huong Le Thi Thien <huongltt35@fpt.com>
Co-authored-by: Nam Pham Dinh Thanh <nampdt@fpt.com>
Co-authored-by: Vu Dam Tuan <vudt15@fpt.com>
Co-authored-by: Hiep Ha Van <hiephv3@fpt.com>
Co-authored-by: Lam Hoang Van <lamhv7@fpt.com>
Reviewed-on: #7
Co-authored-by: Duy Le Huu <duylh19@fpt.com>

tests/unit/test_agent_result.py

@@ -0,0 +1,101 @@
+"""R04-T03 (a) — unit tests for the value a finished turn returns.
+
+Two callers need different things out of one turn today:
+``ui/chat_panel.py::_finalize_turn`` wants the message list, while
+``core/task_executors.py::_run_agent`` returns a
+``(answer_text, timed_out, incomplete_reason)`` tuple assembled by hand. This
+type is what both read instead, so "what happened in that turn?" has one answer
+with names on it.
+"""
+
+from __future__ import annotations
+
+from dataclasses import FrozenInstanceError
+
+import pytest
+from cowork_local.domain.agents.agent_event import PlanStep, TurnCompletedEvent
+from cowork_local.domain.agents.agent_result import AgentResult
+
+
+def test_result_rejects_mutation() -> None:
+    result = AgentResult(steps_used=1)
+
+    with pytest.raises(FrozenInstanceError):
+        result.steps_used = 2
+
+
+def test_messages_are_frozen_into_a_tuple() -> None:
+    live = [{"role": "user", "content": "hi"}]
+
+    result = AgentResult(messages=live)
+    live.append({"role": "assistant", "content": "later"})
+
+    assert result.messages == ({"role": "user", "content": "hi"},)
+
+
+def test_final_text_is_the_last_non_empty_assistant_message() -> None:
+    # A turn ends on a tool message often enough (cancelled mid-loop) that the
+    # answer cannot simply be messages[-1].
+    result = AgentResult(messages=[
+        {"role": "assistant", "content": "first pass"},
+        {"role": "assistant", "content": "the answer"},
+        {"role": "tool", "tool_call_id": "c", "name": "read_file", "content": "..."},
+    ])
+
+    assert result.final_text == "the answer"
+
+
+def test_final_text_skips_a_blank_assistant_message() -> None:
+    result = AgentResult(messages=[
+        {"role": "assistant", "content": "the answer"},
+        {"role": "assistant", "content": "   "},
+    ])
+
+    assert result.final_text == "the answer"
+
+
+def test_final_text_is_empty_when_the_model_never_answered() -> None:
+    assert AgentResult(messages=[{"role": "user", "content": "hi"}]).final_text == ""
+
+
+def test_a_plain_finished_turn_is_ok() -> None:
+    assert AgentResult(messages=[{"role": "assistant", "content": "done"}]).ok is True
+
+
+def test_a_cancelled_turn_is_not_ok() -> None:
+    assert AgentResult(cancelled=True).ok is False
+
+
+def test_a_failed_turn_is_not_ok_and_keeps_its_message() -> None:
+    result = AgentResult(error="SecurityBlocked: nope")
+
+    assert result.ok is False
+    assert result.error == "SecurityBlocked: nope"
+
+
+def test_hitting_the_step_ceiling_is_reported_separately_from_cancelling() -> None:
+    # "Stopped because the safety limit was reached" and "the user pressed Stop"
+    # need different wording in the transcript, so they stay separate flags.
+    result = AgentResult(budget_exhausted=True, steps_used=30)
+
+    assert result.budget_exhausted is True
+    assert result.cancelled is False
+
+
+def test_result_converts_to_the_turn_completed_event() -> None:
+    result = AgentResult(
+        messages=[{"role": "assistant", "content": "done"}],
+        steps_used=3, cancelled=False, budget_exhausted=True,
+    )
+
+    assert result.to_turn_completed_event() == TurnCompletedEvent(
+        final_text="done", steps_used=3, cancelled=False, budget_exhausted=True)
+
+
+def test_plan_steps_are_frozen_into_a_tuple() -> None:
+    steps = [PlanStep(title="Draft", status="done")]
+
+    result = AgentResult(plan_steps=steps)
+    steps.append(PlanStep(title="Review"))
+
+    assert result.plan_steps == (PlanStep(title="Draft", status="done"),)