refactor: move SafeFileHistory to module level + add regression tests

- Promote _SafeFileHistory to module-level SafeFileHistory for testability
- Add 5 regression tests: surrogates, normal text, emoji, mixed CJK, multi-surrogates

Made-with: Cursor

nanobot/cli/commands.py

@@ -33,6 +33,19 @@ from rich.table import Table
 from rich.text import Text
 
 from nanobot import __logo__, __version__
+
+
+class SafeFileHistory(FileHistory):
+    """FileHistory subclass that sanitizes surrogate characters on write.
+
+    On Windows, special Unicode input (emoji, mixed-script) can produce
+    surrogate characters that crash prompt_toolkit's file write.
+    See issue #2846.
+    """
+
+    def store_string(self, string: str) -> None:
+        safe = string.encode("utf-8", errors="surrogateescape").decode("utf-8", errors="replace")
+        super().store_string(safe)
 from nanobot.cli.stream import StreamRenderer, ThinkingSpinner
 from nanobot.config.paths import get_workspace_path, is_default_workspace
 from nanobot.config.schema import Config
@@ -118,17 +131,8 @@ def _init_prompt_session() -> None:
     history_file = get_cli_history_path()
     history_file.parent.mkdir(parents=True, exist_ok=True)
 
-    # Wrap FileHistory to sanitize surrogate characters on write.
-    # Without this, special Unicode input (emoji, mixed-script) crashes
-    # prompt_toolkit's history file write on Windows with UnicodeEncodeError.
-    # See issue #2846.
-    class _SafeFileHistory(FileHistory):
-        def store_string(self, string: str) -> None:
-            safe = string.encode("utf-8", errors="surrogateescape").decode("utf-8", errors="replace")
-            super().store_string(safe)
-
     _PROMPT_SESSION = PromptSession(
-        history=_SafeFileHistory(str(history_file)),
+        history=SafeFileHistory(str(history_file)),
         enable_open_in_editor=False,
         multiline=False,  # Enter submits (single line mode)
     )