fix(node-adapter): handle utf-8 characters split across stream chunks (#1496)

## Summary
- use `StringDecoder` to decode stream content safely across chunk
boundaries
- add tests for JSON and text bodies with UTF-8 characters split between
chunks

  ## Problem
Direct `chunk.toString()` decoding can corrupt UTF-8 characters when
multi-byte sequences are split across stream chunks.

  ## Testing
  - added unit tests for chunk-boundary UTF-8 decoding in JSON body
  - added unit tests for chunk-boundary UTF-8 decoding in text body

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Improved UTF‑8 decoding for streamed request bodies so multi‑byte
characters split across chunks are decoded correctly and incomplete
final bytes are handled gracefully (replacement character used).

* **Tests**
* Added tests validating JSON and plain‑text payloads with multi‑byte
UTF‑8 characters split across stream boundaries, including an incomplete
final byte scenario.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: zxhyc131 <zxhyc131@mercallure.com>
Co-authored-by: Dinh Le <dinwwwh@gmail.com>

Author: 3 people

packages/standard-server-node/src/body.test.ts

@@ -16,6 +16,29 @@ beforeEach(() => {
   vi.clearAllMocks()
 })
 
+function createChunkedRequest(contentType: string, chunks: Buffer[]): IncomingMessage {
+  const request = Readable.from(chunks) as IncomingMessage
+  request.headers = {
+    'content-type': contentType,
+  }
+  return request
+}
+
+function splitBufferInsideCharacter(text: string, splitCharacter: string): Buffer[] {
+  const buffer = Buffer.from(text)
+  const splitBytes = Buffer.from(splitCharacter)
+  const splitIndex = buffer.indexOf(splitBytes)
+
+  if (splitIndex === -1) {
+    throw new Error(`split character not found: ${splitCharacter}`)
+  }
+
+  return [
+    buffer.subarray(0, splitIndex + 1),
+    buffer.subarray(splitIndex + 1),
+  ]
+}
+
 describe('toStandardBody', () => {
   it('undefined', async () => {
     let standardBody: StandardBody = {} as any
@@ -51,6 +74,42 @@ describe('toStandardBody', () => {
     expect(standardBody).toEqual({ foo: 'bar' })
   })
 
+  it('json with utf-8 characters split across chunk boundaries', async () => {
+    const original = {
+      json: {
+        text: '滚滚长江东逝水',
+      },
+    }
+
+    const chunks = splitBufferInsideCharacter(JSON.stringify(original), '江')
+    const request = createChunkedRequest('application/json', chunks)
+
+    const standardBody = await toStandardBody(request)
+
+    expect(standardBody).toEqual(original)
+  })
+
+  it('text with utf-8 characters split across chunk boundaries', async () => {
+    const original = '海内存知己,天涯若比邻'
+    const chunks = splitBufferInsideCharacter(original, '存')
+    const request = createChunkedRequest('text/plain', chunks)
+
+    const standardBody = await toStandardBody(request)
+
+    expect(standardBody).toBe(original)
+  })
+
+  it('text with utf-8 characters split across chunk boundaries end with incomplete utf8', async () => {
+    const original = '海内存知己,天涯若比邻'
+    const chunks = splitBufferInsideCharacter(original, '存')
+    const incompleteUtf8 = Buffer.from([230, 181])
+    const request = createChunkedRequest('text/plain', [...chunks, incompleteUtf8])
+
+    const standardBody = await toStandardBody(request)
+
+    expect(standardBody).toBe(`${original}�`)
+  })
+
   it('json but empty body', async () => {
     let standardBody: StandardBody = {} as any
 

packages/standard-server-node/src/body.ts

@@ -1,7 +1,7 @@
 import type { StandardBody, StandardHeaders } from '@orpc/standard-server'
-import type { Buffer } from 'node:buffer'
 import type { ToEventIteratorOptions, ToEventStreamOptions } from './event-iterator'
 import type { NodeHttpRequest } from './types'
+import { Buffer } from 'node:buffer'
 import { Readable } from 'node:stream'
 import { isAsyncIteratorObject, parseEmptyableJSON, runWithSpan, stringifyJSON } from '@orpc/shared'
 import { flattenHeader, generateContentDisposition, getFilenameFromContentDisposition } from '@orpc/standard-server'
@@ -114,13 +114,16 @@ function _streamToFormData(stream: Readable, contentType: string): Promise<FormD
 }
 
 async function _streamToString(stream: Readable): Promise<string> {
-  let string = ''
+  const decoder = new TextDecoder()
+  let text = ''
 
   for await (const chunk of stream) {
-    string += chunk.toString()
+    text += decoder.decode(Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk), { stream: true })
   }
 
-  return string
+  text += decoder.decode()
+
+  return text
 }
 
 async function _streamToFile(stream: Readable, fileName: string, contentType: string): Promise<File> {