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.

cli.py

@@ -12526,7 +12526,18 @@ def main(
                     ):
                         cli.session_id = cli.agent.session_id
                     response = result.get("final_response", "") if isinstance(result, dict) else str(result)
-                    if response:
+                    # Surface backend errors that produced no visible output
+                    # (e.g. invalid model slug → provider 4xx). Mirrors the
+                    # interactive CLI path. Write to stderr so piped stdout
+                    # stays clean for automation wrappers.
+                    if (
+                        not response
+                        and isinstance(result, dict)
+                        and result.get("error")
+                        and (result.get("failed") or result.get("partial"))
+                    ):
+                        print(f"Error: {result['error']}", file=sys.stderr)
+                    elif response:
                         print(response)
                     # Session ID goes to stderr so piped stdout is clean.
                     print(f"\nsession_id: {cli.session_id}", file=sys.stderr)