fix: sanitize surrogate characters from clipboard paste to prevent UnicodeEncodeError (#3624)

Pasting text from rich-text editors (Google Docs, Word, etc.) can inject
lone surrogate characters (U+D800..U+DFFF) that are invalid UTF-8.
The OpenAI SDK serializes messages with ensure_ascii=False, then encodes
to UTF-8 for the HTTP body — surrogates crash this with:
  UnicodeEncodeError: 'utf-8' codec can't encode character '\udce2'

Three-layer fix:
1. Primary: sanitize user_message at the top of run_conversation()
2. CLI: sanitize in chat() before appending to conversation_history
3. Safety net: catch UnicodeEncodeError in the API error handler,
   sanitize the entire messages list in-place, and retry once.
   Also exclude UnicodeEncodeError from is_local_validation_error
   so it doesn't get classified as non-retryable.

Includes 14 new tests covering the sanitization helpers and the
integration with run_conversation().

cli.py

@@ -5534,6 +5534,13 @@ class HermesCLI:
             except Exception as e:
                 logging.debug("@ context reference expansion failed: %s", e)
 
+        # Sanitize surrogate characters that can arrive via clipboard paste from
+        # rich-text editors (Google Docs, Word, etc.).  Lone surrogates are invalid
+        # UTF-8 and crash JSON serialization in the OpenAI SDK.
+        if isinstance(message, str):
+            from run_agent import _sanitize_surrogates
+            message = _sanitize_surrogates(message)
+
         # Add user message to history
         self.conversation_history.append({"role": "user", "content": message})