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_internal_test.go

@@ -0,0 +1,164 @@
+package taskname
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/codersdk"
+	"github.com/coder/coder/v2/testutil"
+)
+
+func TestGenerateFallback(t *testing.T) {
+	t.Parallel()
+
+	taskName := generateFallback()
+	err := codersdk.NameValid(taskName.Name)
+	require.NoErrorf(t, err, "expected fallback to be valid workspace name, instead found %s", taskName.Name)
+	require.NotEmpty(t, taskName.DisplayName)
+}
+
+func TestGenerateFromPrompt(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                string
+		prompt              string
+		expectError         bool
+		expectedName        string
+		expectedDisplayName string
+	}{
+		{
+			name:        "EmptyPrompt",
+			prompt:      "",
+			expectError: true,
+		},
+		{
+			name:        "OnlySpaces",
+			prompt:      "     ",
+			expectError: true,
+		},
+		{
+			name:        "OnlySpecialCharacters",
+			prompt:      "!@#$%^&*()",
+			expectError: true,
+		},
+		{
+			name:                "UppercasePrompt",
+			prompt:              "BUILD MY APP",
+			expectError:         false,
+			expectedName:        "build-my-app",
+			expectedDisplayName: "BUILD MY APP",
+		},
+		{
+			name:                "PromptWithApostrophes",
+			prompt:              "fix user's dashboard",
+			expectError:         false,
+			expectedName:        "fix-users-dashboard",
+			expectedDisplayName: "Fix user's dashboard",
+		},
+		{
+			name:                "LongPrompt",
+			prompt:              strings.Repeat("a", 100),
+			expectError:         false,
+			expectedName:        strings.Repeat("a", 27),
+			expectedDisplayName: "A" + strings.Repeat("a", 62) + "…",
+		},
+		{
+			name:                "PromptWithMultipleSpaces",
+			prompt:              "build    my    app",
+			expectError:         false,
+			expectedName:        "build-my-app",
+			expectedDisplayName: "Build    my    app",
+		},
+		{
+			name:                "PromptWithNewlines",
+			prompt:              "build\nmy\napp",
+			expectError:         false,
+			expectedName:        "build-my-app",
+			expectedDisplayName: "Build my app",
+		},
+		{
+			name:                "TruncatesLongPromptAtWordBoundary",
+			prompt:              "implement real-time notifications dashboard",
+			expectError:         false,
+			expectedName:        "implement-real-time",
+			expectedDisplayName: "Implement real-time notifications dashboard",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			taskName, err := generateFromPrompt(tc.prompt)
+
+			if tc.expectError {
+				require.Error(t, err)
+				return
+			}
+
+			require.NoError(t, err)
+
+			// Validate task name
+			require.Contains(t, taskName.Name, fmt.Sprintf("%s-", tc.expectedName))
+			require.NoError(t, codersdk.NameValid(taskName.Name))
+
+			// Validate task display name
+			require.NotEmpty(t, taskName.DisplayName)
+			require.Equal(t, tc.expectedDisplayName, taskName.DisplayName)
+		})
+	}
+}
+
+func TestGenerateFromAnthropic(t *testing.T) {
+	t.Parallel()
+
+	apiKey := getAnthropicAPIKeyFromEnv()
+	if apiKey == "" {
+		t.Skip("Skipping test as ANTHROPIC_API_KEY not set")
+	}
+
+	tests := []struct {
+		name   string
+		prompt string
+	}{
+		{
+			name:   "SimplePrompt",
+			prompt: "Create a finance planning app",
+		},
+		{
+			name:   "TechnicalPrompt",
+			prompt: "Debug authentication middleware for OAuth2",
+		},
+		{
+			name:   "ShortPrompt",
+			prompt: "Fix bug",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			ctx := testutil.Context(t, testutil.WaitShort)
+
+			taskName, err := generateFromAnthropic(ctx, tc.prompt, apiKey, getAnthropicModelFromEnv())
+			require.NoError(t, err)
+
+			t.Log("Task name:", taskName.Name)
+			t.Log("Task display name:", taskName.DisplayName)
+
+			// Validate task name
+			require.NotEmpty(t, taskName.DisplayName)
+			require.NoError(t, codersdk.NameValid(taskName.Name))
+
+			// Validate display name
+			require.NotEmpty(t, taskName.DisplayName)
+			require.NotEqual(t, "task-unnamed", taskName.Name)
+			require.NotEqual(t, "Task Unnamed", taskName.DisplayName)
+		})
+	}
+}