test: add tests for stop agent feature

- TestAgentContextStop: 9 unit tests for the stop() method
  - clears paused state, streaming agent, calls kill_process
  - logs info message, preserves conversation history
  - safe when not running, differs from nudge/reset behavior
- TestStopApiHandler: 3 tests for the /stop API endpoint
  - returns success, raises without context id, actually stops agent

tests/test_stop_agent.py

@@ -0,0 +1,125 @@
+import sys
+import threading
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+from flask import Flask
+
+PROJECT_ROOT = Path(__file__).resolve().parents[1]
+if str(PROJECT_ROOT) not in sys.path:
+    sys.path.insert(0, str(PROJECT_ROOT))
+
+from agent import AgentContext
+from initialize import initialize_agent
+from api.stop import Stop
+
+
+@pytest.fixture
+def ctx():
+    ctxid = "ctx-stop-test"
+    context = AgentContext(config=initialize_agent(), id=ctxid, set_current=False)
+    yield context
+    AgentContext.remove(ctxid)
+
+
+class TestAgentContextStop:
+    """Unit tests for AgentContext.stop() method."""
+
+    def test_stop_clears_paused_state(self, ctx):
+        ctx.paused = True
+        ctx.stop()
+        assert ctx.paused is False
+
+    def test_stop_clears_streaming_agent(self, ctx):
+        ctx.streaming_agent = MagicMock()
+        ctx.stop()
+        assert ctx.streaming_agent is None
+
+    def test_stop_calls_kill_process(self, ctx):
+        with patch.object(ctx, "kill_process") as mock_kill:
+            ctx.stop()
+            mock_kill.assert_called_once()
+
+    def test_stop_logs_info_message(self, ctx):
+        initial_log_count = len(ctx.log.logs)
+        ctx.stop()
+        assert len(ctx.log.logs) > initial_log_count
+        last_log = ctx.log.logs[-1]
+        assert last_log.type == "info"
+        assert "stopped" in last_log.content.lower()
+
+    def test_stop_preserves_conversation_history(self, ctx):
+        ctx.log.log(type="user", heading="test", content="hello world")
+        log_count_before = len(ctx.log.logs)
+        ctx.stop()
+        # Should have original logs + the "stopped" info log
+        assert len(ctx.log.logs) == log_count_before + 1
+
+    def test_stop_does_not_reset_agent(self, ctx):
+        original_agent = ctx.agent0
+        ctx.stop()
+        assert ctx.agent0 is original_agent
+
+    def test_stop_when_not_running_is_safe(self, ctx):
+        """Calling stop when nothing is running should not raise."""
+        ctx.task = None
+        ctx.paused = False
+        ctx.streaming_agent = None
+        ctx.stop()  # should not raise
+        assert ctx.paused is False
+
+    def test_stop_differs_from_nudge(self, ctx):
+        """Stop should NOT restart the agent (unlike nudge)."""
+        ctx.stop()
+        # After stop, task should be None (no new task started)
+        # nudge() would set self.task to a new communicate() call
+        assert ctx.task is None
+
+    def test_stop_differs_from_reset(self, ctx):
+        """Stop should NOT clear the log (unlike reset)."""
+        ctx.log.log(type="user", heading="test", content="preserved")
+        ctx.stop()
+        # reset() calls log.reset() which clears everything
+        assert len(ctx.log.logs) >= 2  # original + stopped message
+
+
+class TestStopApiHandler:
+    """Tests for the api/stop.py endpoint."""
+
+    @pytest.mark.asyncio
+    async def test_stop_api_returns_success(self, ctx):
+        app = Flask("stop-api-test")
+        app.secret_key = "test-secret"
+        lock = threading.RLock()
+
+        handler = Stop(app, lock)
+        result = await handler.process({"context": ctx.id}, None)
+
+        assert result["message"] == "Agent stopped."
+        assert result["context"] == ctx.id
+
+    @pytest.mark.asyncio
+    async def test_stop_api_raises_without_context_id(self):
+        app = Flask("stop-api-test")
+        app.secret_key = "test-secret"
+        lock = threading.RLock()
+
+        handler = Stop(app, lock)
+        with pytest.raises(Exception, match="No context id provided"):
+            await handler.process({"context": ""}, None)
+
+    @pytest.mark.asyncio
+    async def test_stop_api_actually_stops_agent(self, ctx):
+        ctx.paused = True
+        ctx.streaming_agent = MagicMock()
+
+        app = Flask("stop-api-test")
+        app.secret_key = "test-secret"
+        lock = threading.RLock()
+
+        handler = Stop(app, lock)
+        await handler.process({"context": ctx.id}, None)
+
+        assert ctx.paused is False
+        assert ctx.streaming_agent is None