fix(coding-agent): decode pi.exec output across UTF-8 chunk boundaries (#11326)

`execCommand()` (the function behind `pi.exec()`) decoded each stdout
and stderr chunk on its own with `data.toString()`. When a multi-byte
UTF-8 character was split across two pipe chunks, each half was turned
into U+FFFD, so the result was corrupted. For example, `"你好"` came back
as `"���好"` while the command still exited with code 0.

This change gives stdout and stderr each their own
`StringDecoder("utf8")`. A decoder holds back incomplete bytes until the
rest of the character arrives. Both decoders are flushed with `end()`
before the promise resolves, on both the success path and the error
path. `waitForChildProcess()` can resolve without a stream `end` event,
so the decoders are flushed there and not in an `end` handler. The
return shape, exit codes, and kill behavior are unchanged.

**Testing**
- Added a regression test to `packages/coding-agent/test/exec.test.ts`.
The child process splits `"你好"` across two writes on both stdout and
stderr. The test fails without the fix and passes with it: `node
../../node_modules/vitest/dist/cli.js --run test/exec.test.ts` reports 3
passed.
- `npm run check` passes.
- Reproduction from the issue: a child that writes the first byte of
`"你好"` and then the rest 100 ms later. Before the fix the result was
`{"stdout":"���好",...}`. After the fix it is
`{"stdout":"你好","stderr":"","code":0,"killed":false}`.

Fixes #11312

Requested by @davidbrai

---------

Co-authored-by: triagent <triagent@localhost>
Co-authored-by: David Brailovsky <davidbrai@gmail.com>

packages/coding-agent/src/core/exec.ts

@@ -3,6 +3,7 @@
  */
 
 import { spawn } from "node:child_process";
+import { StringDecoder } from "node:string_decoder";
 import { waitForChildProcess } from "../utils/child-process.ts";
 
 /**
@@ -46,6 +47,10 @@ export async function execCommand(
 
 		let stdout = "";
 		let stderr = "";
+		// Decode across chunks: a multi-byte UTF-8 character can be split between two chunks,
+		// and decoding each chunk separately would corrupt it (earendil-works/pi#11312).
+		const stdoutDecoder = new StringDecoder("utf8");
+		const stderrDecoder = new StringDecoder("utf8");
 		let killed = false;
 		let timeoutId: NodeJS.Timeout | undefined;
 
@@ -79,11 +84,11 @@ export async function execCommand(
 		}
 
 		proc.stdout?.on("data", (data) => {
-			stdout += data.toString();
+			stdout += stdoutDecoder.write(data);
 		});
 
 		proc.stderr?.on("data", (data) => {
-			stderr += data.toString();
+			stderr += stderrDecoder.write(data);
 		});
 
 		// Wait for process termination without hanging on inherited stdio handles
@@ -91,6 +96,8 @@ export async function execCommand(
 		waitForChildProcess(proc)
 			.then((code) => {
 				if (timeoutId) clearTimeout(timeoutId);
+				stdout += stdoutDecoder.end();
+				stderr += stderrDecoder.end();
 				if (options?.signal) {
 					options.signal.removeEventListener("abort", killProcess);
 				}
@@ -98,6 +105,8 @@ export async function execCommand(
 			})
 			.catch((error: Error) => {
 				if (timeoutId) clearTimeout(timeoutId);
+				stdout += stdoutDecoder.end();
+				stderr += stderrDecoder.end();
 				if (options?.signal) {
 					options.signal.removeEventListener("abort", killProcess);
 				}