test: add comprehensive tests for stdio discovery

- Add test fixture for broken syntax error handling
- Refactor tests to use unified DiscoveryResult interface
- Parameterize tests with nested loops to test all cases with both discovery methods
- Add error handling tests for both HTTP and stdio discovery
- All stdio discovery tests passing (16/16)

Author: taeold

scripts/bin-test/sources/broken-syntax/index.js

@@ -0,0 +1,6 @@
+const functions = require("firebase-functions");
+
+// This will cause a syntax error
+exports.broken = functions.https.onRequest((request, response) => {
+  response.send("Hello from Firebase!"
+}); // Missing closing parenthesis

scripts/bin-test/sources/broken-syntax/package-lock.json

@@ -0,0 +1,7 @@
+{
+  "name": "broken-syntax",
+  "main": "index.js",
+  "dependencies": {
+    "firebase-functions": "file:../../../.."
+  }
+}