refactor: remove deprecated AITaskPromptParameterName constant (#21023)

This removes the deprecated AITaskPromptParameterName constant and all
backward compatibility code that was added for v2.28.

- Remove AITaskPromptParameterName constant from codersdk/aitasks.go
- Remove backward compatibility code in coderd/aitasks.go that populated
  the "AI Prompt" parameter for templates that defined it
- Remove the backward compatibility test (OK AIPromptBackCompat)
- Update dbfake to no longer set the AI Prompt parameter
- Remove AITaskPromptParameterName from frontend TypeScript types
- Remove preset prompt read-only feature from TaskPrompt component
- Update docs to reflect that pre-2.28 definition is no longer supported

Task prompts are now exclusively stored in the tasks.prompt database
column, as introduced in the migration that added the tasks table.

codersdk/aitasks.go

@@ -10,23 +10,8 @@ import (
 
 	"github.com/google/uuid"
 	"golang.org/x/xerrors"
-
-	"github.com/coder/terraform-provider-coder/v2/provider"
 )
 
-// AITaskPromptParameterName is the name of the parameter used to pass prompts
-// to AI tasks.
-//
-// Deprecated: This constant is deprecated and maintained only for backwards
-// compatibility with older templates. Task prompts are now stored directly
-// in the tasks.prompt database column. New code should access prompts via
-// the Task.InitialPrompt field returned from task endpoints.
-//
-// This constant will be removed in a future major version. Templates should
-// not rely on this parameter name, as the backend will continue to create it
-// automatically for compatibility but reads from tasks.prompt.
-const AITaskPromptParameterName = provider.TaskPromptParameterName
-
 // CreateTaskRequest represents the request to create a new task.
 type CreateTaskRequest struct {
 	TemplateVersionID       uuid.UUID `json:"template_version_id" format:"uuid"`

codersdk/toolsdk/toolsdk_test.go

@@ -1017,7 +1017,6 @@ func TestTools(t *testing.T) {
 			ProvisionApply: echo.ApplyComplete,
 			ProvisionGraph: []*proto.Response{
 				{Type: &proto.Response_Graph{Graph: &proto.GraphComplete{
-					Parameters: []*proto.RichParameter{{Name: "AI Prompt", Type: "string"}},
 					HasAiTasks: true,
 				}}},
 			},