test(poll): add concurrency test for single-poller capacity control

- Introduce TaskRunner interface to decouple Poller from concrete run.Runner
- Add TestPoller_ConcurrencyLimitedByCapacity verifying max concurrent
  tasks respects capacity and FetchTask is never called concurrently
- Mock runner respects context cancellation for proper shutdown testing

internal/app/poll/poller_test.go

@@ -6,6 +6,8 @@ package poll
 import (
 	"context"
 	"errors"
+	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -105,3 +107,96 @@ func TestPoller_CalculateInterval(t *testing.T) {
 		})
 	}
 }
+
+// atomicMax atomically updates target to max(target, val).
+func atomicMax(target *atomic.Int64, val int64) {
+	for {
+		old := target.Load()
+		if val <= old || target.CompareAndSwap(old, val) {
+			break
+		}
+	}
+}
+
+type mockRunner struct {
+	delay          time.Duration
+	running        atomic.Int64
+	maxConcurrent  atomic.Int64
+	totalCompleted atomic.Int64
+}
+
+func (m *mockRunner) Run(ctx context.Context, _ *runnerv1.Task) error {
+	atomicMax(&m.maxConcurrent, m.running.Add(1))
+	select {
+	case <-time.After(m.delay):
+	case <-ctx.Done():
+	}
+	m.running.Add(-1)
+	m.totalCompleted.Add(1)
+	return nil
+}
+
+// TestPoller_ConcurrencyLimitedByCapacity verifies that with capacity=3 and
+// 6 available tasks, at most 3 tasks run concurrently, and FetchTask is
+// never called concurrently (single poller).
+func TestPoller_ConcurrencyLimitedByCapacity(t *testing.T) {
+	const (
+		capacity   = 3
+		totalTasks = 6
+		taskDelay  = 50 * time.Millisecond
+	)
+
+	var (
+		tasksReturned  atomic.Int64
+		fetchConcur    atomic.Int64
+		maxFetchConcur atomic.Int64
+	)
+
+	cli := mocks.NewClient(t)
+	cli.On("FetchTask", mock.Anything, mock.Anything).Return(
+		func(_ context.Context, _ *connect_go.Request[runnerv1.FetchTaskRequest]) (*connect_go.Response[runnerv1.FetchTaskResponse], error) {
+			atomicMax(&maxFetchConcur, fetchConcur.Add(1))
+			defer fetchConcur.Add(-1)
+
+			n := tasksReturned.Add(1)
+			if n <= totalTasks {
+				return connect_go.NewResponse(&runnerv1.FetchTaskResponse{
+					Task: &runnerv1.Task{Id: n},
+				}), nil
+			}
+			return connect_go.NewResponse(&runnerv1.FetchTaskResponse{}), nil
+		},
+	)
+
+	runner := &mockRunner{delay: taskDelay}
+
+	cfg, err := config.LoadDefault("")
+	require.NoError(t, err)
+	cfg.Runner.Capacity = capacity
+	cfg.Runner.FetchInterval = 10 * time.Millisecond
+	cfg.Runner.FetchIntervalMax = 10 * time.Millisecond
+
+	poller := New(cfg, cli, runner)
+
+	var wg sync.WaitGroup
+	wg.Go(poller.Poll)
+
+	require.Eventually(t, func() bool {
+		return runner.totalCompleted.Load() >= totalTasks
+	}, 2*time.Second, 10*time.Millisecond, "all tasks should complete")
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+	err = poller.Shutdown(ctx)
+	require.NoError(t, err)
+	wg.Wait()
+
+	assert.LessOrEqual(t, runner.maxConcurrent.Load(), int64(capacity),
+		"concurrent running tasks must not exceed capacity")
+	assert.GreaterOrEqual(t, runner.maxConcurrent.Load(), int64(2),
+		"with 6 tasks and capacity 3, at least 2 should overlap")
+	assert.Equal(t, int64(1), maxFetchConcur.Load(),
+		"FetchTask must never be called concurrently (single poller)")
+	assert.Equal(t, int64(totalTasks), runner.totalCompleted.Load(),
+		"all tasks should have been executed")
+}