fix(tui_gateway): restore _content_display_text helper

Bb/gui had dropped the helper but the orchestrator code merged from main
still calls it (_inflight_text, _message_preview). Re-add the definition
verbatim from main so session.create / _start_inflight_turn don't crash
with NameError on first prompt submit.

tui_gateway/server.py

@@ -2421,6 +2421,36 @@ def _enrich_with_attached_images(user_text: str, image_paths: list[str]) -> str:
     return text or "What do you see in this image?"
 
 
+def _content_display_text(content: Any) -> str:
+    if content is None:
+        return ""
+    if isinstance(content, str):
+        return content
+    if isinstance(content, (int, float)):
+        return str(content)
+    if isinstance(content, list):
+        parts = []
+        for part in content:
+            text = _content_display_text(part).strip()
+            if text:
+                parts.append(text)
+        return "\n".join(parts)
+    if isinstance(content, dict):
+        kind = content.get("type")
+        if kind in {"text", "input_text", "output_text"}:
+            return str(content.get("text") or content.get("content") or "")
+        if kind in {"image_url", "input_image", "image"}:
+            return "[image]"
+        if kind in {"input_audio", "audio"}:
+            return "[audio]"
+        if kind:
+            return f"[{kind}]"
+        if "text" in content:
+            return str(content.get("text") or "")
+        return "[structured content]"
+    return str(content)
+
+
 def _coerce_message_text(content: Any) -> str:
     """Render ``message['content']`` as a plain string for transport.