[Refactor] Rework Environment Variable Checks in Email-Rendering Pipeline (#1161)

### Context
Via the email-monitor and sentry traces, we found out that we were
running the "without fallback" path in production. This was because the
right environment variables weren't populated in Vercel, but it exposed
how our system let this slide. This occurred because the check we did
for fallback vs non-fallback routes relied on checking whether a certain
environment variable was empty.

### Summary of Changes
We now use a sentinel value for the check instead of an empty value.
Additionally, we remove default values from the `getEnvVariable` checks
to ensure they throw if not populated. We also guard against the
sentinel value being used in production

### Config Changes
We need to update and make sure the vercel sandbox environment variables
are populated in production.

Author: nams1570

apps/backend/.env.development

@@ -53,6 +53,7 @@ STACK_ENABLE_HARDCODED_PASSKEY_CHALLENGE_FOR_TESTING=yes
 STACK_INTEGRATION_CLIENTS_CONFIG=[{"client_id": "neon-local", "client_secret": "neon-local-secret", "id_token_signed_response_alg": "ES256", "redirect_uris": ["http://localhost:30000/api/v2/identity/authorize", "http://localhost:30000/api/v2/auth/authorize"]}, {"client_id": "custom-local", "client_secret": "custom-local-secret", "id_token_signed_response_alg": "ES256", "redirect_uris": ["http://localhost:30000/api/v2/identity/authorize", "http://localhost:30000/api/v2/auth/authorize"]}]
 CRON_SECRET=mock_cron_secret
 STACK_FREESTYLE_API_KEY=mock_stack_freestyle_key
+STACK_VERCEL_SANDBOX_TOKEN=vercel_sandbox_disabled_for_local_development
 STACK_OPENAI_API_KEY=mock_openai_api_key
 STACK_STRIPE_SECRET_KEY=sk_test_mockstripekey
 STACK_STRIPE_WEBHOOK_SECRET=mock_stripe_webhook_secret

apps/backend/src/lib/js-execution.tsx

@@ -57,9 +57,9 @@ function createVercelSandboxEngine(): JsEngine {
   return {
     name: 'vercel-sandbox',
     execute: async (code: string, options: ExecuteJavascriptOptions): Promise<ExecuteResult> => {
-      const teamId = getEnvVariable("STACK_VERCEL_SANDBOX_TEAM_ID", "");
-      const projectId = getEnvVariable("STACK_VERCEL_SANDBOX_PROJECT_ID", "");
-      const token = getEnvVariable("STACK_VERCEL_SANDBOX_TOKEN", "");
+      const teamId = getEnvVariable("STACK_VERCEL_SANDBOX_TEAM_ID");
+      const projectId = getEnvVariable("STACK_VERCEL_SANDBOX_PROJECT_ID");
+      const token = getEnvVariable("STACK_VERCEL_SANDBOX_TOKEN");
 
       const sandbox = await Sandbox.create({
         resources: { vcpus: 2 },
@@ -139,18 +139,18 @@ export async function executeJavascript(code: string, options: ExecuteJavascript
     }
   }, async () => {
 
-    if (getEnvVariable("STACK_VERCEL_SANDBOX_TOKEN","") != "") {
-      if (!getNodeEnvironment().includes("prod")) {
-        throw new StackAssertionError("STACK_VERCEL_SANDBOX_TOKEN is set in non-production environment. We do not use Vercel Sandbox in non-production environments.");
-      }
-
+    if (getEnvVariable("STACK_VERCEL_SANDBOX_TOKEN") != "vercel_sandbox_disabled_for_local_development") {
       const shouldSanityTest = Math.random() < 0.05;
       if (shouldSanityTest) {
         runAsynchronouslyAndWaitUntil(runSanityTest(code, options));
       }
 
       return await runWithFallback(code, options);
     } else {
+      if (getNodeEnvironment().includes("prod")) {
+        throw new StackAssertionError("STACK_VERCEL_SANDBOX_TOKEN is set to the disabled sentinel value in production. Please configure a real Vercel Sandbox token.");
+      }
+
       return await runWithoutFallback(code, options);
     }
   });