fix: handle UnicodeEncodeError with ASCII codec (#6843)

Broaden the UnicodeEncodeError recovery to handle systems with ASCII-only
locale (LANG=C, Chromebooks) where ANY non-ASCII character causes encoding
failure, not just lone surrogates.

Changes:
- Add _strip_non_ascii() and _sanitize_messages_non_ascii() helpers that
  strip all non-ASCII characters from message content, name, and tool_calls
- Update the UnicodeEncodeError handler to detect ASCII codec errors and
  fall back to non-ASCII sanitization after surrogate check fails
- Sanitize tool_calls arguments and name fields (not just content)
- Fix bare .encode() in cli.py suspend handler to use explicit utf-8
- Add comprehensive test suite (17 tests)

tests/run_agent/test_unicode_ascii_codec.py

@@ -0,0 +1,120 @@
+"""Tests for UnicodeEncodeError recovery with ASCII codec.
+
+Covers the fix for issue #6843 — systems with ASCII locale (LANG=C)
+that can't encode non-ASCII characters in API request payloads.
+"""
+
+import pytest
+
+from run_agent import (
+    _strip_non_ascii,
+    _sanitize_messages_non_ascii,
+    _sanitize_messages_surrogates,
+)
+
+
+class TestStripNonAscii:
+    """Tests for _strip_non_ascii helper."""
+
+    def test_ascii_only(self):
+        assert _strip_non_ascii("hello world") == "hello world"
+
+    def test_removes_non_ascii(self):
+        assert _strip_non_ascii("hello ⚕ world") == "hello  world"
+
+    def test_removes_emoji(self):
+        assert _strip_non_ascii("test 🤖 done") == "test  done"
+
+    def test_chinese_chars(self):
+        assert _strip_non_ascii("你好world") == "world"
+
+    def test_empty_string(self):
+        assert _strip_non_ascii("") == ""
+
+    def test_only_non_ascii(self):
+        assert _strip_non_ascii("⚕🤖") == ""
+
+
+class TestSanitizeMessagesNonAscii:
+    """Tests for _sanitize_messages_non_ascii."""
+
+    def test_no_change_ascii_only(self):
+        messages = [{"role": "user", "content": "hello"}]
+        assert _sanitize_messages_non_ascii(messages) is False
+        assert messages[0]["content"] == "hello"
+
+    def test_sanitizes_content_string(self):
+        messages = [{"role": "user", "content": "hello ⚕ world"}]
+        assert _sanitize_messages_non_ascii(messages) is True
+        assert messages[0]["content"] == "hello  world"
+
+    def test_sanitizes_content_list(self):
+        messages = [{
+            "role": "user",
+            "content": [{"type": "text", "text": "hello 🤖"}]
+        }]
+        assert _sanitize_messages_non_ascii(messages) is True
+        assert messages[0]["content"][0]["text"] == "hello "
+
+    def test_sanitizes_name_field(self):
+        messages = [{"role": "tool", "name": "⚕tool", "content": "ok"}]
+        assert _sanitize_messages_non_ascii(messages) is True
+        assert messages[0]["name"] == "tool"
+
+    def test_sanitizes_tool_calls(self):
+        messages = [{
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [{
+                "id": "call_1",
+                "type": "function",
+                "function": {
+                    "name": "read_file",
+                    "arguments": '{"path": "⚕test.txt"}'
+                }
+            }]
+        }]
+        assert _sanitize_messages_non_ascii(messages) is True
+        assert messages[0]["tool_calls"][0]["function"]["arguments"] == '{"path": "test.txt"}'
+
+    def test_handles_non_dict_messages(self):
+        messages = ["not a dict", {"role": "user", "content": "hello"}]
+        assert _sanitize_messages_non_ascii(messages) is False
+
+    def test_empty_messages(self):
+        assert _sanitize_messages_non_ascii([]) is False
+
+    def test_multiple_messages(self):
+        messages = [
+            {"role": "system", "content": "⚕ System prompt"},
+            {"role": "user", "content": "Hello 你好"},
+            {"role": "assistant", "content": "Hi there!"},
+        ]
+        assert _sanitize_messages_non_ascii(messages) is True
+        assert messages[0]["content"] == " System prompt"
+        assert messages[1]["content"] == "Hello "
+        assert messages[2]["content"] == "Hi there!"
+
+
+class TestSurrogateVsAsciiSanitization:
+    """Test that surrogate and ASCII sanitization work independently."""
+
+    def test_surrogates_still_handled(self):
+        """Surrogates are caught by _sanitize_messages_surrogates, not _non_ascii."""
+        msg_with_surrogate = "test \ud800 end"
+        messages = [{"role": "user", "content": msg_with_surrogate}]
+        assert _sanitize_messages_surrogates(messages) is True
+        assert "\ud800" not in messages[0]["content"]
+        assert "\ufffd" in messages[0]["content"]
+
+    def test_ascii_codec_strips_all_non_ascii(self):
+        """ASCII codec case: all non-ASCII is stripped, not replaced."""
+        messages = [{"role": "user", "content": "test ⚕🤖你好 end"}]
+        assert _sanitize_messages_non_ascii(messages) is True
+        # All non-ASCII chars removed; spaces around them collapse
+        assert messages[0]["content"] == "test  end"
+
+    def test_no_surrogates_returns_false(self):
+        """When no surrogates present, _sanitize_messages_surrogates returns False."""
+        messages = [{"role": "user", "content": "hello ⚕ world"}]
+        assert _sanitize_messages_surrogates(messages) is False