coder terraform provider: modify `coder_ai_task` to add app_id and prompt fields

[johnstcn]

Modify the coder_ai_task resource to introduce two new read-only (computed) fields:

• Task ID (task_id) from CODER_TASK_ID
• Task Prompt (prompt) from CODER_TASK_PROMPT

Add the required fields to the coder_ai_task resource:

• app_id

Deprecate the sidebar_app field of the coder_ai_task resource in favour of app_id.

As a compatibility measure, we can add logic to set app_id from sidebar_app.id.

Example usage:

resource "coder_agent" "main" { ... }
resource "coder_app" "claude_code_web" { ... }

resource "coder_ai_task" "task" {
  // task_id [read-only]
  // prompt [read-only]
  app_id = coder_app.claude_code_web.id
  // alternatively, for backward compat:
  //sidebar_app {
  //  id = coder_app.claude_code_web.id
  //}
}