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

codersdk/aitasks.go

@@ -35,6 +35,7 @@ type CreateTaskRequest struct {
 	TemplateVersionPresetID uuid.UUID `json:"template_version_preset_id,omitempty" format:"uuid"`
 	Input                   string    `json:"input"`
 	Name                    string    `json:"name,omitempty"`
+	DisplayName             string    `json:"display_name,omitempty"`
 }
 
 // CreateTask creates a new task.
@@ -128,6 +129,7 @@ type Task struct {
 	OwnerName               string                   `json:"owner_name" table:"owner name"`
 	OwnerAvatarURL          string                   `json:"owner_avatar_url,omitempty" table:"owner avatar url"`
 	Name                    string                   `json:"name" table:"name,default_sort"`
+	DisplayName             string                   `json:"display_name" table:"display_name"`
 	TemplateID              uuid.UUID                `json:"template_id" format:"uuid" table:"template id"`
 	TemplateVersionID       uuid.UUID                `json:"template_version_id" format:"uuid" table:"template version id"`
 	TemplateName            string                   `json:"template_name" table:"template name"`