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

cli/testdata/coder_server_--help.golden

@@ -911,7 +911,7 @@ TEMPLATE BUILDER OPTIONS:
           Disable the template builder feature for guided template creation.
           When disabled, all /api/v2/templatebuilder/* endpoints return 404.
 
-      --template-builder-registry-url string, $CODER_TEMPLATE_BUILDER_REGISTRY_URL (default: https://registry.coder.com)
+      --template-builder-registry-url string, $CODER_TEMPLATE_BUILDER_REGISTRY_URL (default: registry.coder.com)
           The base URL of the module registry used by the template builder for
           module source paths.
 

cli/testdata/server-config.yaml.golden

@@ -1145,5 +1145,5 @@ templateBuilder:
   disabled: false
   # The base URL of the module registry used by the template builder for module
   # source paths.
-  # (default: https://registry.coder.com, type: string)
-  registryURL: https://registry.coder.com
+  # (default: registry.coder.com, type: string)
+  registryURL: registry.coder.com