fix: set codersdk.Task current_state during task initialization (#20692)

## Problem

With the new tasks data model, a task starts with an `initializing`
status. However, the API returns `current_state: null` to represent the
agent state, causing the frontend to display "No message available".
This PR updates `codersdk.Task` to return a `current_state` when the
task is initializing with meaningful messages about what's happening
during task initialization.

**Previous message**

<img width="2764" height="288" alt="Screenshot 2025-11-07 at 09 06 13"
src="https://github.com/user-attachments/assets/feec9f15-91ca-4378-8565-5f9de062d11a"
/>

**New message**

<img width="2726" height="226" alt="Screenshot 2025-11-12 at 11 00 15"
src="https://github.com/user-attachments/assets/2f9bee3e-7ac4-4382-b1c3-1d06bbc2906e"
/>

## Changes

- Populate `current_state` with descriptive initialization messages when
task status is `initializing` and no valid app status exists for the
current build
- **dbfake**: Fix `WorkspaceBuild` builder to properly handle
pending/running jobs by linking tasks without requiring agent/app
resources

**Note:** UI Storybook changes to reflect these new messages will be
addressed in a follow-up PR.

Closes: https://github.com/coder/internal/issues/1063

coderd/aitasks_internal_test.go

@@ -0,0 +1,223 @@
+package coderd
+
+import (
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/util/ptr"
+	"github.com/coder/coder/v2/codersdk"
+)
+
+func TestDeriveTaskCurrentState_Unit(t *testing.T) {
+	t.Parallel()
+
+	now := time.Now()
+	tests := []struct {
+		name               string
+		task               database.Task
+		agentLifecycle     *codersdk.WorkspaceAgentLifecycle
+		appHealth          *codersdk.WorkspaceAppHealth
+		latestAppStatus    *codersdk.WorkspaceAppStatus
+		latestBuild        codersdk.WorkspaceBuild
+		expectCurrentState bool
+		expectedTimestamp  time.Time
+		expectedState      codersdk.TaskState
+		expectedMessage    string
+	}{
+		{
+			name: "NoAppStatus",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusActive,
+			},
+			agentLifecycle:  nil,
+			appHealth:       nil,
+			latestAppStatus: nil,
+			latestBuild: codersdk.WorkspaceBuild{
+				Transition: codersdk.WorkspaceTransitionStart,
+				CreatedAt:  now,
+			},
+			expectCurrentState: false,
+		},
+		{
+			name: "BuildStartTransition_AppStatus_NewerThanBuild",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusActive,
+			},
+			agentLifecycle: nil,
+			appHealth:      nil,
+			latestAppStatus: &codersdk.WorkspaceAppStatus{
+				State:     codersdk.WorkspaceAppStatusStateWorking,
+				Message:   "Task is working",
+				CreatedAt: now.Add(1 * time.Minute),
+			},
+			latestBuild: codersdk.WorkspaceBuild{
+				Transition: codersdk.WorkspaceTransitionStart,
+				CreatedAt:  now,
+			},
+			expectCurrentState: true,
+			expectedTimestamp:  now.Add(1 * time.Minute),
+			expectedState:      codersdk.TaskState(codersdk.WorkspaceAppStatusStateWorking),
+			expectedMessage:    "Task is working",
+		},
+		{
+			name: "BuildStartTransition_StaleAppStatus_OlderThanBuild",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusActive,
+			},
+			agentLifecycle: nil,
+			appHealth:      nil,
+			latestAppStatus: &codersdk.WorkspaceAppStatus{
+				State:     codersdk.WorkspaceAppStatusStateComplete,
+				Message:   "Previous task completed",
+				CreatedAt: now.Add(-1 * time.Minute),
+			},
+			latestBuild: codersdk.WorkspaceBuild{
+				Transition: codersdk.WorkspaceTransitionStart,
+				CreatedAt:  now,
+			},
+			expectCurrentState: false,
+		},
+		{
+			name: "BuildStopTransition",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusActive,
+			},
+			agentLifecycle: nil,
+			appHealth:      nil,
+			latestAppStatus: &codersdk.WorkspaceAppStatus{
+				State:     codersdk.WorkspaceAppStatusStateComplete,
+				Message:   "Task completed before stop",
+				CreatedAt: now.Add(-1 * time.Minute),
+			},
+			latestBuild: codersdk.WorkspaceBuild{
+				Transition: codersdk.WorkspaceTransitionStop,
+				CreatedAt:  now,
+			},
+			expectCurrentState: true,
+			expectedTimestamp:  now.Add(-1 * time.Minute),
+			expectedState:      codersdk.TaskState(codersdk.WorkspaceAppStatusStateComplete),
+			expectedMessage:    "Task completed before stop",
+		},
+		{
+			name: "TaskInitializing_WorkspacePending",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusInitializing,
+			},
+			agentLifecycle:  nil,
+			appHealth:       nil,
+			latestAppStatus: nil,
+			latestBuild: codersdk.WorkspaceBuild{
+				Status:    codersdk.WorkspaceStatusPending,
+				CreatedAt: now,
+			},
+			expectCurrentState: true,
+			expectedTimestamp:  now,
+			expectedState:      codersdk.TaskStateWorking,
+			expectedMessage:    "Workspace is pending",
+		},
+		{
+			name: "TaskInitializing_WorkspaceStarting",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusInitializing,
+			},
+			agentLifecycle:  nil,
+			appHealth:       nil,
+			latestAppStatus: nil,
+			latestBuild: codersdk.WorkspaceBuild{
+				Status:    codersdk.WorkspaceStatusStarting,
+				CreatedAt: now,
+			},
+			expectCurrentState: true,
+			expectedTimestamp:  now,
+			expectedState:      codersdk.TaskStateWorking,
+			expectedMessage:    "Workspace is starting",
+		},
+		{
+			name: "TaskInitializing_AgentConnecting",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusInitializing,
+			},
+			agentLifecycle:  ptr.Ref(codersdk.WorkspaceAgentLifecycleCreated),
+			appHealth:       nil,
+			latestAppStatus: nil,
+			latestBuild: codersdk.WorkspaceBuild{
+				Status:    codersdk.WorkspaceStatusRunning,
+				CreatedAt: now,
+			},
+			expectCurrentState: true,
+			expectedTimestamp:  now,
+			expectedState:      codersdk.TaskStateWorking,
+			expectedMessage:    "Agent is connecting",
+		},
+		{
+			name: "TaskInitializing_AgentStarting",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusInitializing,
+			},
+			agentLifecycle:  ptr.Ref(codersdk.WorkspaceAgentLifecycleStarting),
+			appHealth:       nil,
+			latestAppStatus: nil,
+			latestBuild: codersdk.WorkspaceBuild{
+				Status:    codersdk.WorkspaceStatusRunning,
+				CreatedAt: now,
+			},
+			expectCurrentState: true,
+			expectedTimestamp:  now,
+			expectedState:      codersdk.TaskStateWorking,
+			expectedMessage:    "Agent is starting",
+		},
+		{
+			name: "TaskInitializing_AppInitializing",
+			task: database.Task{
+				ID:     uuid.New(),
+				Status: database.TaskStatusInitializing,
+			},
+			agentLifecycle:  ptr.Ref(codersdk.WorkspaceAgentLifecycleReady),
+			appHealth:       ptr.Ref(codersdk.WorkspaceAppHealthInitializing),
+			latestAppStatus: nil,
+			latestBuild: codersdk.WorkspaceBuild{
+				Status:    codersdk.WorkspaceStatusRunning,
+				CreatedAt: now,
+			},
+			expectCurrentState: true,
+			expectedTimestamp:  now,
+			expectedState:      codersdk.TaskStateWorking,
+			expectedMessage:    "App is initializing",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			ws := codersdk.Workspace{
+				LatestBuild:     tt.latestBuild,
+				LatestAppStatus: tt.latestAppStatus,
+			}
+
+			currentState := deriveTaskCurrentState(tt.task, ws, tt.agentLifecycle, tt.appHealth)
+
+			if tt.expectCurrentState {
+				require.NotNil(t, currentState)
+				assert.Equal(t, tt.expectedTimestamp.UTC(), currentState.Timestamp.UTC())
+				assert.Equal(t, tt.expectedState, currentState.State)
+				assert.Equal(t, tt.expectedMessage, currentState.Message)
+			} else {
+				assert.Nil(t, currentState)
+			}
+		})
+	}
+}