feat: add display name field for tasks (#20856)

## Problem

Tasks currently only expose a machine-friendly name field (e.g.
`task-python-debug-a1b2`), but this value is primarily an identifier
rather than a clean, descriptive label. We need a separate
display-friendly name for use in the UI.

This PR introduces a new `display_name` field and updates the task-name
generation flow. The Claude system prompt was updated to return valid
JSON with both `name` and `display_name`. The name generation logic
follows a fallback chain (Anthropic > prompt sanitization > random
fallback). To make task names more closely resemble their display names,
the legacy `task-` prefix has been removed. For context, PR
https://github.com/coder/coder/pull/20834 introduced a small Task icon
to the workspace list to help identify workspaces associated to tasks.

## Changes

- Database migration: Added `display_name` column to tasks table
- Updated system prompt to generate both task name and display name as
valid JSON
- Task name generation now follows a fallback chain: Anthropic > prompt
sanitization > random fallback
- Removed `task-` prefix from task names to allow more descriptive names
- Note: PR https://github.com/coder/coder/pull/20834 adds a Task icon to
workspaces in the workspace list to distinguish task-created workspaces

**Note:** UI changes will be addressed in a follow-up PR

Related to: https://github.com/coder/coder/issues/20801

coderd/taskname/taskname_test.go

@@ -15,42 +15,51 @@ const (
 	anthropicEnvVar = "ANTHROPIC_API_KEY"
 )
 
-func TestGenerateFallback(t *testing.T) {
-	t.Parallel()
-
-	name := taskname.GenerateFallback()
-	err := codersdk.NameValid(name)
-	require.NoErrorf(t, err, "expected fallback to be valid workspace name, instead found %s", name)
-}
-
-func TestGenerateTaskName(t *testing.T) {
-	t.Parallel()
-
-	t.Run("Fallback", func(t *testing.T) {
-		t.Parallel()
+func TestGenerate(t *testing.T) {
+	t.Run("FromPrompt", func(t *testing.T) {
+		// Ensure no API key in env for this test
+		t.Setenv("ANTHROPIC_API_KEY", "")
 
 		ctx := testutil.Context(t, testutil.WaitShort)
 
-		name, err := taskname.Generate(ctx, "Some random prompt")
-		require.ErrorIs(t, err, taskname.ErrNoAPIKey)
-		require.Equal(t, "", name)
+		taskName := taskname.Generate(ctx, testutil.Logger(t), "Create a finance planning app")
+
+		// Should succeed via prompt sanitization
+		require.NoError(t, codersdk.NameValid(taskName.Name))
+		require.Contains(t, taskName.Name, "create-a-finance-planning-")
+		require.NotEmpty(t, taskName.DisplayName)
+		require.Equal(t, "Create a finance planning app", taskName.DisplayName)
 	})
 
-	t.Run("Anthropic", func(t *testing.T) {
-		t.Parallel()
-
+	t.Run("FromAnthropic", func(t *testing.T) {
 		apiKey := os.Getenv(anthropicEnvVar)
 		if apiKey == "" {
 			t.Skipf("Skipping test as %s not set", anthropicEnvVar)
 		}
 
+		// Set API key for this test
+		t.Setenv("ANTHROPIC_API_KEY", apiKey)
+
 		ctx := testutil.Context(t, testutil.WaitShort)
 
-		name, err := taskname.Generate(ctx, "Create a finance planning app", taskname.WithAPIKey(apiKey))
-		require.NoError(t, err)
-		require.NotEqual(t, "", name)
+		taskName := taskname.Generate(ctx, testutil.Logger(t), "Create a finance planning app")
 
-		err = codersdk.NameValid(name)
-		require.NoError(t, err, "name should be valid")
+		// Should succeed with Claude-generated names
+		require.NoError(t, codersdk.NameValid(taskName.Name))
+		require.NotEmpty(t, taskName.DisplayName)
+	})
+
+	t.Run("Fallback", func(t *testing.T) {
+		// Ensure no API key
+		t.Setenv("ANTHROPIC_API_KEY", "")
+
+		ctx := testutil.Context(t, testutil.WaitShort)
+
+		// Use a prompt that can't be sanitized (only special chars)
+		taskName := taskname.Generate(ctx, testutil.Logger(t), "!@#$%^&*()")
+
+		// Should fall back to random name
+		require.NoError(t, codersdk.NameValid(taskName.Name))
+		require.NotEmpty(t, taskName.DisplayName)
 	})
 }