fix(sandbox): binary mode subprocess to fix Windows pipe deadlock (§5.16)

- Remove text=True, encoding='utf-8' from Popen — causes pipe buffer
  deadlock when MCP server redirects stdout for JSON-RPC
- Decode stdout/stderr manually with .decode('utf-8', errors='replace')
- Pattern matches §5.16: Popen(PIPE) + communicate(timeout) + binary
- Sandbox tests: 34/34 passed. execute_script in MCP runtime: needs
  server restart to pick up this change.

Author: MSCodeBase Agent

src/core/sandbox/executor.py

@@ -336,25 +336,28 @@ def execute_sandboxed(
         creationflags = subprocess.CREATE_NO_WINDOW
 
     try:
+        # Binary mode to avoid pipe buffer deadlock on Windows (§5.16)
         proc = subprocess.Popen(
             [sys.executable, "-c", script],
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
             cwd=cwd,
             env=env,
             creationflags=creationflags,
-            text=True,
-            encoding="utf-8",
         )
-        stdout, stderr = proc.communicate(timeout=timeout)
+        raw_out, raw_err = proc.communicate(timeout=timeout)
+        stdout = raw_out.decode("utf-8", errors="replace") if raw_out else ""
+        stderr = raw_err.decode("utf-8", errors="replace") if raw_err else ""
         exit_code = proc.returncode
         timed_out = False
     except subprocess.TimeoutExpired:
         try:
             proc.kill()
         except OSError:
             pass  # Process may have already exited between timeout and kill
-        stdout, stderr = proc.communicate()
+        raw_out, raw_err = proc.communicate()
+        stdout = raw_out.decode("utf-8", errors="replace") if raw_out else ""
+        stderr = raw_err.decode("utf-8", errors="replace") if raw_err else ""
         exit_code = -1
         timed_out = True
     except Exception as e: