fix(est): reject oversized attachments early

resources/tester/helper_est_native.cjs

@@ -2,7 +2,7 @@ const fs = require("node:fs");
 const path = require("node:path");
 const vm = require("node:vm");
 const base = path.resolve(__dirname, "../../tools/est-test");
-const context = vm.createContext({ structuredClone, console });
+const context = vm.createContext({ structuredClone, console, Blob });
 for (const file of [
   "native-math.js",
   "native-formula.js",
@@ -19,6 +19,7 @@ for (const file of [
   "native-resources.js",
   "native-schedule.js",
   "native-issues.js",
+  "native-attachments.js",
   "native-model.js",
   "native-c3r.js",
   "native-exchange.js",

resources/tester/test_est_attachments.py

@@ -3,6 +3,21 @@ import subprocess
 from pathlib import Path
 
 
+def test_attachment_size_prediction_includes_utf8_and_base64_overhead():
+    result = run(
+        "math",
+        """(()=>{
+      const p=EstNative.empty();p.name='한글';const file={name:'파일.bin',type:'',size:4};
+      const expected={...p,id:'0'.repeat(24),attachments:[{id:'0'.repeat(36),...file,data:'AAECAw=='}]};
+      return {predicted:EstAttachments.requiredBytes(p,[file]),actual:new Blob([JSON.stringify(expected)]).size,
+        large:EstAttachments.requiredBytes(p,[{name:'large',type:'',size:210000000}]),count:p.attachments.length};
+    })()""",
+    )
+    assert result["predicted"] == result["actual"]
+    assert result["large"] > 256 * 1024 * 1024
+    assert result["count"] == 0
+
+
 def test_attachment_validation_keeps_valid_data_and_rejects_malformed_content():
     result = run(
         "math",