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_test.go

@@ -240,14 +240,18 @@ func TestTasks(t *testing.T) {
 		assert.NotNil(t, updated.CurrentState, "current state should not be nil")
 		assert.Equal(t, "all done", updated.CurrentState.Message)
 		assert.Equal(t, codersdk.TaskStateComplete, updated.CurrentState.State)
+		previousCurrentState := updated.CurrentState
 
 		// Start the workspace again
 		coderdtest.MustTransitionWorkspace(t, client, task.WorkspaceID.UUID, codersdk.WorkspaceTransitionStop, codersdk.WorkspaceTransitionStart)
 
-		// Verify that the status from the previous build is no longer present
+		// Verify that the status from the previous build has been cleared
+		// and replaced by the agent initialization status.
 		updated, err = exp.TaskByID(ctx, task.ID)
 		require.NoError(t, err)
-		assert.Nil(t, updated.CurrentState, "current state should be nil")
+		assert.NotEqual(t, previousCurrentState, updated.CurrentState)
+		assert.Equal(t, codersdk.TaskStateWorking, updated.CurrentState.State)
+		assert.NotEqual(t, "all done", updated.CurrentState.Message)
 	})
 
 	t.Run("Delete", func(t *testing.T) {