feat: template builder backend fixes (DEVEX-287) (#26432)

Part of the Template Builder wizard PR stack.

## Backend fixes

1. **Registry URL scheme fix**: Default
`CODER_TEMPLATE_BUILDER_REGISTRY_URL` was `https://registry.coder.com`
but Terraform module registry addresses must be scheme-less. Changed to
`registry.coder.com`.

2. **Sensitive variable defaults**: Module `.tf.tmpl` files for
claude-code, aider, amazon-q had sensitive `variable` blocks without
`default`, causing `terraform plan` to fail during template import. Also
fixed the `templatebuildermodulegen` script.

3. **Auto-quote string variables**: The backend now accepts raw string
values from callers and wraps them in HCL quotes automatically.
Previously callers were required to send pre-quoted HCL literals, which
is not a reasonable API contract.

---
> [!NOTE]
> Generated by Coder Agents on behalf of @jeremyruppel

coderd/templatebuilder/compose_test.go

@@ -93,7 +93,7 @@ func TestCompose(t *testing.T) {
 				{
 					ID: "git-clone",
 					Variables: map[string]string{
-						"url": `"https://github.com/coder/coder"`,
+						"url": "https://github.com/coder/coder",
 					},
 				},
 			},
@@ -178,7 +178,7 @@ func TestCompose(t *testing.T) {
 				{
 					ID: "code-server",
 					Variables: map[string]string{
-						"nonexistent_var": `"value"`,
+						"nonexistent_var": "value",
 					},
 				},
 			},
@@ -216,7 +216,7 @@ func TestCompose(t *testing.T) {
 				{
 					ID: "code-server",
 					Variables: map[string]string{
-						"folder": `"${var.evil}"`,
+						"folder": "${var.evil}",
 					},
 				},
 			},