feat: add status watcher to MCP server (#18320)

This is meant to complement the existing task reporter since the LLM
does not call it reliably.

It also includes refactoring to use the common agent flags/env vars.

cli/cliutil/queue_test.go

@@ -0,0 +1,110 @@
+package cliutil_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/cli/cliutil"
+)
+
+func TestQueue(t *testing.T) {
+	t.Parallel()
+
+	t.Run("DropsFirst", func(t *testing.T) {
+		t.Parallel()
+
+		q := cliutil.NewQueue[int](10)
+		require.Equal(t, 0, q.Len())
+
+		for i := 0; i < 20; i++ {
+			err := q.Push(i)
+			require.NoError(t, err)
+			if i < 10 {
+				require.Equal(t, i+1, q.Len())
+			} else {
+				require.Equal(t, 10, q.Len())
+			}
+		}
+
+		val, ok := q.Pop()
+		require.True(t, ok)
+		require.Equal(t, 10, val)
+		require.Equal(t, 9, q.Len())
+	})
+
+	t.Run("Pop", func(t *testing.T) {
+		t.Parallel()
+
+		q := cliutil.NewQueue[int](10)
+		for i := 0; i < 5; i++ {
+			err := q.Push(i)
+			require.NoError(t, err)
+		}
+
+		// No blocking, should pop immediately.
+		for i := 0; i < 5; i++ {
+			val, ok := q.Pop()
+			require.True(t, ok)
+			require.Equal(t, i, val)
+		}
+
+		// Pop should block until the next push.
+		go func() {
+			err := q.Push(55)
+			assert.NoError(t, err)
+		}()
+
+		item, ok := q.Pop()
+		require.True(t, ok)
+		require.Equal(t, 55, item)
+	})
+
+	t.Run("Close", func(t *testing.T) {
+		t.Parallel()
+
+		q := cliutil.NewQueue[int](10)
+
+		done := make(chan bool)
+		go func() {
+			_, ok := q.Pop()
+			done <- ok
+		}()
+
+		q.Close()
+
+		require.False(t, <-done)
+
+		_, ok := q.Pop()
+		require.False(t, ok)
+
+		err := q.Push(10)
+		require.Error(t, err)
+	})
+
+	t.Run("WithPredicate", func(t *testing.T) {
+		t.Parallel()
+
+		q := cliutil.NewQueue[int](10)
+		q.WithPredicate(func(n int) (int, bool) {
+			if n == 2 {
+				return n, false
+			}
+			return n + 1, true
+		})
+
+		for i := 0; i < 5; i++ {
+			err := q.Push(i)
+			require.NoError(t, err)
+		}
+
+		got := []int{}
+		for i := 0; i < 4; i++ {
+			val, ok := q.Pop()
+			require.True(t, ok)
+			got = append(got, val)
+		}
+		require.Equal(t, []int{1, 2, 4, 5}, got)
+	})
+}