Pull the 10 pure sanitization/repair helpers (\_sanitize_surrogates,
\_sanitize_structure_surrogates, \_sanitize_messages_surrogates,
\_escape_invalid_chars_in_json_strings, \_repair_tool_call_arguments,
\_strip_non_ascii, \_sanitize_messages_non_ascii, \_sanitize_tools_non_ascii,
\_strip_images_from_messages, \_sanitize_structure_non_ascii) and the
\_SURROGATE_RE constant out of run_agent.py into a new module.
These are stateless byte-walking helpers with no AIAgent dependency.
Backward compatibility: run_agent re-exports every name via a single
import block, so existing 'from run_agent import _sanitize_surrogates'
imports in tests and cli.py keep working unchanged. Same pattern the
file already uses for _summarize_user_message_for_log (codex_responses_adapter).
run_agent.py: 16077 -> 15682 lines (-395).
