fix(tui): surface backend error as visible text when final_response is empty (#21245)

When the provider rejects a request (e.g. invalid model slug like
'--provider nous --model kimi-k2.6' where the valid slug is
'moonshotai/kimi-k2.6'), run_conversation() returns
{failed: True, error: <detail>, final_response: None}. The TUI gateway
and one-shot CLI mode both dropped the error on the floor and emitted
an empty turn, so the user saw a blank response with no indication
that anything went wrong.

Mirror the interactive CLI's existing pattern (cli.py:9832): when
final_response is empty AND (failed|partial) is set AND error is
populated, surface 'Error: <detail>' as the visible text. Leaves
the None-with-no-error path and the '(empty)' sentinel path
untouched — an empty successful turn still renders empty, and
existing sentinel handlers keep owning their lane.

Reported by @counterposition in PR #20873; taking a minimal fix
rather than the broader structured-failure refactor proposed there.

tui_gateway/server.py

@@ -3137,6 +3137,18 @@ def _run_prompt_submit(rid, sid: str, session: dict, text: Any) -> None:
                     if result.get("interrupted")
                     else "error" if result.get("error") else "complete"
                 )
+                # When the backend produced no visible response AND reported a
+                # real error (e.g. invalid model slug → provider 4xx), surface
+                # that error as the visible text instead of shipping an empty
+                # turn to Ink. Mirrors classic CLI behavior at cli.py where
+                # (failed|partial) + no final_response → "Error: <detail>".
+                # Leaves the None-with-no-error path untouched: an empty
+                # successful turn still renders as empty, and the existing
+                # "(empty)" sentinel handling stays in its own lane.
+                if (not raw) and result.get("error") and (
+                    result.get("failed") or result.get("partial")
+                ):
+                    raw = f"Error: {result.get('error')}"
                 lr = result.get("last_reasoning")
                 if isinstance(lr, str) and lr.strip():
                     last_reasoning = lr.strip()