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/modules/aider/aider.tf.tmpl

@@ -2,6 +2,7 @@
 variable "api_key" {
   description = "API key for the selected AI provider. This will be set as the appropriate environment variable based on the provider."
   type        = string
+  default     = ""
   sensitive   = true
 }
 module "aider" {

coderd/templatebuilder/modules/amazon-q/amazon-q.tf.tmpl

@@ -2,6 +2,7 @@
 variable "auth_tarball" {
   description = "Base64 encoded, zstd compressed tarball of a pre-authenticated ~/.local/share/amazon-q directory."
   type        = string
+  default     = ""
   sensitive   = true
 }
 module "amazon-q" {

coderd/templatebuilder/modules/claude-code/claude-code.tf.tmpl

@@ -2,6 +2,7 @@
 variable "claude_code_oauth_token" {
   description = "OAuth token passed to Claude Code via the CLAUDE_CODE_OAUTH_TOKEN env var. Generate one with `claude setup-token`."
   type        = string
+  default     = ""
   sensitive   = true
 }
 module "claude-code" {