refactor(coder-labs/sourcegraph-amp): support terraform provider coder v2.12.0 (#495)

## Description

Updates the module to use the new version of the agentapi module for
Coder 2.28

## Type of Change

- [ ] New module
- [ ] New template
- [ ] Bug fix
- [x] Feature/enhancement
- [ ] Documentation
- [ ] Other

## Module Information

**Path:** `registry/coder-labs/modules/sourcegraph-amp`  
**New version:** `v3.0.0`  
**Breaking change:** [x] Yes [ ] No

## Testing & Validation

- [x] Tests pass (`bun test`)
- [x] Code formatted (`bun fmt`)
- [ ] Changes tested locally

## Related Issues

- https://github.com/coder/internal/issues/1065

## Related PRs

- https://github.com/coder/registry/pull/485

---------

Co-authored-by: DevCats <christofer@coder.com>

registry/coder-labs/modules/sourcegraph-amp/README.md

@@ -13,7 +13,7 @@ Run [Amp CLI](https://ampcode.com/) in your workspace to access Sourcegraph's AI
 ```tf
 module "amp-cli" {
   source           = "registry.coder.com/coder-labs/sourcegraph-amp/coder"
-  version          = "2.1.0"
+  version          = "3.0.0"
   agent_id         = coder_agent.example.id
   amp_api_key      = var.amp_api_key
   install_amp      = true
@@ -48,7 +48,7 @@ variable "amp_api_key" {
 module "amp-cli" {
   count              = data.coder_workspace.me.start_count
   source             = "registry.coder.com/coder-labs/sourcegraph-amp/coder"
-  amp_version        = "2.1.0"
+  amp_version        = "3.0.0"
   agent_id           = coder_agent.example.id
   amp_api_key        = var.amp_api_key # recommended for tasks usage
   workdir            = "/home/coder/project"

registry/coder-labs/modules/sourcegraph-amp/main.test.ts

@@ -110,6 +110,7 @@ describe("amp", async () => {
     const { id } = await setup({
       skipAmpMock: true,
       moduleVariables: {
+        install_via_npm: "true",
         amp_version: "0.0.1755964909-g31e083",
       },
     });

registry/coder-labs/modules/sourcegraph-amp/main.tf

@@ -4,7 +4,7 @@ terraform {
   required_providers {
     coder = {
       source  = "coder/coder"
-      version = ">= 2.7"
+      version = ">= 2.12"
     }
     external = {
       source  = "hashicorp/external"
@@ -220,7 +220,7 @@ locals {
 
 module "agentapi" {
   source  = "registry.coder.com/coder/agentapi/coder"
-  version = "1.2.0"
+  version = "2.0.0"
 
   agent_id             = var.agent_id
   folder               = local.workdir
@@ -268,4 +268,6 @@ module "agentapi" {
   EOT
 }
 
-
+output "task_app_id" {
+  value = module.agentapi.task_app_id
+}