Migrate tests to Terraform for jetbrains, zed, and code-server (#307)

## Summary
- Introduces Terraform native tests (`terraform test`) alongside
existing Bun tests
- Migrates tests for modules: jetbrains, zed, and code-server
- Removes Bun test files for these migrated modules only
- Adds repo-wide test runner script for Terraform tests
- Updates docs and new-module sample to reflect Terraform tests

## Transition plan
- Mixed mode: Other modules retain Bun tests; CI should run both Bun and
Terraform tests temporarily
- Follow the linked epic to migrate remaining modules

## Test plan
- Run: `./scripts/terraform_test_all.sh` (passes locally)
- Bun tests still available for non-migrated modules

## Affected paths
- registry/coder/modules/jetbrains/jetbrains.tftest.hcl
- registry/coder/modules/zed/zed.tftest.hcl
- registry/coder/modules/code-server/code-server.tftest.hcl
- scripts/terraform_test_all.sh
- examples/modules/MODULE_NAME.tftest.hcl
- CONTRIBUTING.md


Contributes to #308

scripts/terraform_test_all.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Find all directories that contain any .tftest.hcl files and run terraform test in each
+
+run_dir() {
+  local dir="$1"
+  echo "==> Running terraform test in $dir"
+  (cd "$dir" && terraform init -upgrade -input=false -no-color > /dev/null && terraform test -no-color -verbose)
+}
+
+mapfile -t test_dirs < <(find . -type f -name "*.tftest.hcl" -print0 | xargs -0 -I{} dirname {} | sort -u)
+
+if [[ ${#test_dirs[@]} -eq 0 ]]; then
+  echo "No .tftest.hcl tests found."
+  exit 0
+fi
+
+status=0
+for d in "${test_dirs[@]}"; do
+  if ! run_dir "$d"; then
+    status=1
+  fi
+done
+
+exit $status