agent: add PromptFIFO unit tests

Tests cover FIFO ordering, empty-pop behaviour, and concurrent push safety.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/agent/fifo_test.go

@@ -0,0 +1,60 @@
+package agent
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestPromptFIFO_BasicOrder(t *testing.T) {
+	var f PromptFIFO
+	f.Push("a")
+	f.Push("b")
+	f.Push("c")
+
+	for _, want := range []string{"a", "b", "c"} {
+		got, ok := f.Pop()
+		if !ok {
+			t.Fatalf("Pop() = _, false; want %q", want)
+		}
+		if got != want {
+			t.Errorf("Pop() = %q; want %q", got, want)
+		}
+	}
+
+	if got, ok := f.Pop(); ok {
+		t.Errorf("Pop() on empty = %q, true; want \"\", false", got)
+	}
+}
+
+func TestPromptFIFO_EmptyPop(t *testing.T) {
+	var f PromptFIFO
+	if _, ok := f.Pop(); ok {
+		t.Error("Pop() on zero-value FIFO returned ok=true")
+	}
+}
+
+func TestPromptFIFO_ConcurrentPushPop(t *testing.T) {
+	var f PromptFIFO
+	const n = 100
+	var wg sync.WaitGroup
+	for i := 0; i < n; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			f.Push("x")
+		}()
+	}
+	wg.Wait()
+
+	count := 0
+	for {
+		_, ok := f.Pop()
+		if !ok {
+			break
+		}
+		count++
+	}
+	if count != n {
+		t.Errorf("after %d concurrent pushes, popped %d items; want %d", n, count, n)
+	}
+}