chore(fastify-sse): add vitest devDependency

Package was running tests via workspace-hoisted vitest but had no
explicit devDependency. Adds vitest ^3.0.0 for standalone build
correctness.

packages/fastify-sse/package.json

@@ -21,6 +21,9 @@
   "peerDependencies": {
     "fastify": "^5.0.0"
   },
+  "devDependencies": {
+    "vitest": "^3.0.0"
+  },
   "publishConfig": {
     "registry": "http://localhost:3300/api/packages/bytelyst/npm/"
   }