Remove all *_preview zap.String/Any fields from OpenAI-compatible log
calls (chat_handler, responses_handler, stream) so that prompt, content,
reasoning, source, and delta values are never written in plain to
operational logs.
Replace preview fields with non-content metadata:
- prompt_len, content_len, reasoning_len, delta_len, source_len
- message_count, content_len, finish_reason, tool_call_count
Also remove the now-unused previewString helper from types.go.
Add log_safety_test.go with regression tests that:
- verify no *_preview fields exist on any log line
- verify operational metadata fields are retained for observability
- verify logOpenAICompatStreamOutput does not emit delta_preview
