Isolate the stdio server's stdin and stdout from handler subprocesses (#3117)

tests/interaction/_requirements.py

@@ -3919,9 +3919,12 @@ REQUIREMENTS: dict[str, Requirement] = {
         note="Only observable over stdio: stdin/stdout purity is stdio-specific.",
         divergence=Divergence(
             note=(
-                "stdio_server's own writes satisfy this, but it does not redirect or guard sys.stdout: "
-                "handler code that calls print() writes directly to the protocol stream and corrupts the "
-                "framing. The spec MUST is satisfied only as long as application code behaves."
+                "While serving, stdio_server moves the wire to private descriptors and diverts fd 0/1, so "
+                "handler code and its child processes can neither read protocol bytes nor write into the "
+                "stream (pinned by tests/server/test_stdio.py). Remaining gaps: output flushed to stdout "
+                "before the transport enters can still precede the first frame, and the claim is "
+                "best-effort - skipped for explicitly injected streams and for processes without "
+                "normal standard descriptors."
             ),
         ),
     ),

tests/interaction/transports/test_stdio.py

@@ -114,7 +114,8 @@ async def test_stdio_server_writes_one_jsonrpc_message_per_line() -> None:
     """Every `stdio_server` write is one valid JSON-RPC message on its own line.
 
     Each line is newline-terminated with payload newlines JSON-escaped. This proves the
-    transport's own framing; it does not guard `sys.stdout` against handler code (see the
+    transport's own framing over injected streams; the descriptor-level guard that keeps
+    handler code off the wire is pinned by tests/server/test_stdio.py (see the narrowed
     divergence on `transport:stdio:stream-purity`).
     """
     captured = io.StringIO()