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.

docs/ai-coder/ai-bridge/client-config.md

@@ -58,6 +58,8 @@ data "coder_workspace_owner" "me" {}
 
 data "coder_workspace" "me" {}
 
+data "coder_task" "me" {}
+
 resource "coder_agent" "dev" {
   arch = "amd64"
   os   = "linux"
@@ -71,15 +73,21 @@ resource "coder_agent" "dev" {
 
 # See https://registry.coder.com/modules/coder/claude-code for more information
 module "claude-code" {
-  count               = local.has_ai_prompt ? data.coder_workspace.me.start_count : 0
+  count               = data.coder_task.me.enabled ? data.coder_workspace.me.start_count : 0
   source              = "dev.registry.coder.com/coder/claude-code/coder"
-  version             = ">= 3.4.0"
+  version             = ">= 4.0.0"
   agent_id            = coder_agent.dev.id
   workdir             = "/home/coder/project"
   claude_api_key      = data.coder_workspace_owner.me.session_token # Use the Coder session token to authenticate with AI Bridge
-  ai_prompt           = data.coder_parameter.ai_prompt.value
+  ai_prompt           = data.coder_task.me.prompt
   ... # other claude-code configuration
 }
+
+# The coder_ai_task resource associates the task to the app.
+resource "coder_ai_task" "task" {
+  count  = data.coder_task.me.enabled ? data.coder_workspace.me.start_count : 0
+  app_id = module.claude-code[0].task_app_id
+}
 ```
 
 ## External and Desktop Clients

docs/ai-coder/tasks-migration.md

@@ -8,7 +8,8 @@ Prior to Coder version 2.28.0, the definition of a Coder task was different to t
 
 Note that 2 and 3 were generally handled by the `coder/agentapi` Terraform module.
 
-The pre-2.28.0 definition will be supported until the release of 2.29.0. You will need to update your Tasks-enabled templates to continue using Tasks after this release.
+> [!IMPORTANT]
+> The pre-2.28.0 definition is no longer supported as of Coder 2.30.0. You must update your Tasks-enabled templates to use the new format described below.
 
 You can view an [example migration here](https://github.com/coder/coder/pull/20420). Alternatively, follow the steps below:
 
@@ -114,7 +115,7 @@ resource "coder_ai_task" "task" {
 
 In v2.28 and above, the following changes were made:
 
-- The explicitly named "AI Prompt" parameter is deprecated. The task prompt is now available in the `coder_ai_task` resource (provider version 2.12 and above) and `coder_task` data source (provider version 2.13 and above).
+- The explicitly named "AI Prompt" parameter is no longer supported. The task prompt is now available in the `coder_ai_task` resource (provider version 2.12 and above) and `coder_task` data source (provider version 2.13 and above).
 - Modules no longer define the `coder_ai_task` resource. These must be defined explicitly in the template.
 - The `sidebar_app` field of the `coder_ai_task` resource is now deprecated. In its place, use `app_id`.