Add basic cost calculations for testing

package-lock.json

@@ -65,6 +65,7 @@
                 "express": "5.2.1",
                 "express-rate-limit": "8.5.2",
                 "glob": "13.0.6",
+                "gpt-tokenizer": "^3.4.0",
                 "helmet": "8.2.0",
                 "http-errors": "2.0.1",
                 "input-otp": "1.4.2",
@@ -12760,6 +12761,12 @@
                 "url": "https://github.com/sponsors/ljharb"
             }
         },
+        "node_modules/gpt-tokenizer": {
+            "version": "3.4.0",
+            "resolved": "https://registry.npmjs.org/gpt-tokenizer/-/gpt-tokenizer-3.4.0.tgz",
+            "integrity": "sha512-wxFLnhIXTDjYebd9A9pGl3e31ZpSypbpIJSOswbgop5jLte/AsZVDvjlbEuVFlsqZixVKqbcoNmRlFDf6pz/UQ==",
+            "license": "MIT"
+        },
         "node_modules/graceful-fs": {
             "version": "4.2.11",
             "resolved": "https://registry.npmjs.org/graceful-fs/-/graceful-fs-4.2.11.tgz",