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

codersdk/deployment.go

@@ -4776,7 +4776,7 @@ Write out the current server config as YAML to stdout.`,
 			Flag:        "template-builder-registry-url",
 			Env:         "CODER_TEMPLATE_BUILDER_REGISTRY_URL",
 			Value:       &c.TemplateBuilder.RegistryURL,
-			Default:     "https://registry.coder.com",
+			Default:     "registry.coder.com",
 			Group:       &deploymentGroupTemplateBuilder,
 			YAML:        "registryURL",
 		},