fix(cli): suppress OSError EIO on interrupt shutdown

When the user interrupts a long-running task, prompt_toolkit tries to
flush stdout during emergency shutdown.  If stdout is in a broken state
(redirected to /dev/null, pipe closed, terminal gone), the flush raises
`OSError: [Errno 5] Input/output error` which propagates unhandled and
crashes the CLI.

Two defense layers:

1. `_suppress_closed_loop_errors`: add `OSError` with `errno.EIO` to
   the asyncio exception handler, matching the existing pattern for
   `RuntimeError("Event loop is closed")` and `KeyError("is not
   registered")`.

2. Outer `except (KeyError, OSError)` block: add `errno.EIO` check
   before the existing string-match guards, silently suppressing the
   error instead of printing a misleading stdin-related message.

Fixes #13710.

tests/hermes_cli/test_suppress_eio_on_interrupt.py

@@ -0,0 +1,115 @@
+"""Tests for OSError EIO suppression during interrupt shutdown (#13710).
+
+When the user interrupts a running task, prompt_toolkit tries to flush
+stdout during emergency shutdown.  If stdout is already in a broken state
+(redirected to /dev/null, pipe closed, etc.), the flush raises
+``OSError: [Errno 5] Input/output error``.
+
+The ``_suppress_closed_loop_errors`` asyncio exception handler and the
+outer ``except (KeyError, OSError)`` block must both suppress this error
+to prevent a hard crash.
+"""
+
+from __future__ import annotations
+
+import errno
+import os
+from unittest.mock import MagicMock
+
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# _suppress_closed_loop_errors – asyncio exception handler
+# ---------------------------------------------------------------------------
+
+def _make_suppress_fn():
+    """Build a standalone copy of ``_suppress_closed_loop_errors``.
+
+    The real function is defined as a closure inside
+    ``CLI._run_interactive``; we reconstruct an equivalent here so the
+    unit tests don't need a full CLI instance.
+    """
+    def _suppress_closed_loop_errors(loop, context):
+        exc = context.get("exception")
+        if isinstance(exc, RuntimeError) and "Event loop is closed" in str(exc):
+            return
+        if isinstance(exc, KeyError) and "is not registered" in str(exc):
+            return
+        if isinstance(exc, OSError) and getattr(exc, "errno", None) == errno.EIO:
+            return
+        loop.default_exception_handler(context)
+    return _suppress_closed_loop_errors
+
+
+class TestSuppressClosedLoopErrors:
+    """Verify the asyncio exception handler suppresses expected errors."""
+
+    def test_suppresses_event_loop_closed(self):
+        handler = _make_suppress_fn()
+        loop = MagicMock()
+        handler(loop, {"exception": RuntimeError("Event loop is closed")})
+        loop.default_exception_handler.assert_not_called()
+
+    def test_suppresses_key_not_registered(self):
+        handler = _make_suppress_fn()
+        loop = MagicMock()
+        handler(loop, {"exception": KeyError("0 is not registered")})
+        loop.default_exception_handler.assert_not_called()
+
+    def test_suppresses_oserror_eio(self):
+        """OSError with errno.EIO must be suppressed (#13710)."""
+        handler = _make_suppress_fn()
+        loop = MagicMock()
+        exc = OSError(errno.EIO, "Input/output error")
+        handler(loop, {"exception": exc})
+        loop.default_exception_handler.assert_not_called()
+
+    def test_does_not_suppress_oserror_other_errno(self):
+        """OSError with a different errno must still propagate."""
+        handler = _make_suppress_fn()
+        loop = MagicMock()
+        exc = OSError(errno.EACCES, "Permission denied")
+        handler(loop, {"exception": exc})
+        loop.default_exception_handler.assert_called_once()
+
+    def test_does_not_suppress_unrelated_exception(self):
+        """Unrelated exceptions must still propagate."""
+        handler = _make_suppress_fn()
+        loop = MagicMock()
+        handler(loop, {"exception": ValueError("something else")})
+        loop.default_exception_handler.assert_called_once()
+
+    def test_no_exception_key(self):
+        """Context without 'exception' must propagate to default handler."""
+        handler = _make_suppress_fn()
+        loop = MagicMock()
+        handler(loop, {"message": "some log"})
+        loop.default_exception_handler.assert_called_once()
+
+
+# ---------------------------------------------------------------------------
+# Outer except block – EIO handling
+# ---------------------------------------------------------------------------
+
+class TestOuterExceptEIO:
+    """Verify the outer ``except (KeyError, OSError)`` block logic."""
+
+    def test_eio_does_not_reraise(self):
+        """OSError with errno.EIO should be silently suppressed."""
+        exc = OSError(errno.EIO, "Input/output error")
+        # Simulate the condition check from the outer except block:
+        assert isinstance(exc, OSError)
+        assert getattr(exc, "errno", None) == errno.EIO
+
+    def test_bad_file_descriptor_matches(self):
+        """'Bad file descriptor' string should be caught."""
+        exc = OSError(errno.EBADF, "Bad file descriptor")
+        assert "Bad file descriptor" in str(exc)
+
+    def test_other_oserror_reraises(self):
+        """Other OSError variants must not match the EIO guard."""
+        exc = OSError(errno.EACCES, "Permission denied")
+        assert not (getattr(exc, "errno", None) == errno.EIO)
+        assert "is not registered" not in str(exc)
+        assert "Bad file descriptor" not in str(exc)