fix: use Array.from instead of Buffer.from for native processStreamChunk state (#2348)

The napi StreamState fields (utf8Pending, ansiPending) expect plain arrays
(Vec<u8>), not Buffers. Passing Buffer.from() caused 'Given napi value is
not an array on StreamState.utf8Pending' crash on multi-chunk bash output.

Added regression test for multi-chunk state passing.

AI-assisted: This change was authored with Claude (AI pair programming).

packages/native/src/__tests__/stream-process.test.mjs

@@ -0,0 +1,34 @@
+import { test, describe } from "node:test";
+import assert from "node:assert/strict";
+import { processStreamChunk } from "../stream-process/index.ts";
+
+describe("processStreamChunk", () => {
+  test("processes a single chunk without state", () => {
+    const result = processStreamChunk(Buffer.from("hello world\n"));
+    assert.equal(result.text, "hello world\n");
+    assert.ok(Array.isArray(result.state.utf8Pending));
+    assert.ok(Array.isArray(result.state.ansiPending));
+  });
+
+  test("processes multiple chunks passing state between calls", () => {
+    const result1 = processStreamChunk(Buffer.from("first\n"));
+    assert.equal(result1.text, "first\n");
+
+    // This was the crash: passing state back caused
+    // "Given napi value is not an array on StreamState.utf8Pending"
+    // when state arrays were wrapped in Buffer.from() instead of Array.from()
+    const result2 = processStreamChunk(Buffer.from("second\n"), result1.state);
+    assert.equal(result2.text, "second\n");
+
+    const result3 = processStreamChunk(Buffer.from("third\n"), result2.state);
+    assert.equal(result3.text, "third\n");
+  });
+
+  test("state fields are plain arrays, not Buffers", () => {
+    const result = processStreamChunk(Buffer.from("test\n"));
+    assert.ok(Array.isArray(result.state.utf8Pending), "utf8Pending should be a plain array");
+    assert.ok(Array.isArray(result.state.ansiPending), "ansiPending should be a plain array");
+    assert.ok(!(result.state.utf8Pending instanceof Buffer), "utf8Pending should not be a Buffer");
+    assert.ok(!(result.state.ansiPending instanceof Buffer), "ansiPending should not be a Buffer");
+  });
+});

packages/native/src/stream-process/index.ts

@@ -33,8 +33,8 @@ export function processStreamChunk(
   // Convert StreamState arrays to the format napi expects (Vec<u8>)
   const napiState = state
     ? {
-        utf8Pending: Buffer.from(state.utf8Pending),
-        ansiPending: Buffer.from(state.ansiPending),
+        utf8Pending: Array.from(state.utf8Pending),
+        ansiPending: Array.from(state.ansiPending),
       }
     : undefined;