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/database/dbgen/dbgen.go

@@ -14,6 +14,8 @@ import (
 	"testing"
 	"time"
 
+	"cdr.dev/slog"
+
 	"github.com/google/uuid"
 	"github.com/sqlc-dev/pqtype"
 	"github.com/stretchr/testify/require"
@@ -1582,11 +1584,13 @@ func Task(t testing.TB, db database.Store, orig database.TaskTable) database.Tas
 		parameters = json.RawMessage([]byte("{}"))
 	}
 
+	taskName := taskname.Generate(genCtx, slog.Make(), orig.Prompt)
 	task, err := db.InsertTask(genCtx, database.InsertTaskParams{
 		ID:                 takeFirst(orig.ID, uuid.New()),
 		OrganizationID:     orig.OrganizationID,
 		OwnerID:            orig.OwnerID,
-		Name:               takeFirst(orig.Name, taskname.GenerateFallback()),
+		Name:               takeFirst(orig.Name, taskName.Name),
+		DisplayName:        takeFirst(orig.DisplayName, taskName.DisplayName),
 		WorkspaceID:        orig.WorkspaceID,
 		TemplateVersionID:  orig.TemplateVersionID,
 		TemplateParameters: parameters,