Capture validator stdout/stderr; surface tail in error response

tools/hf_space/runner.py

@@ -208,13 +208,29 @@ def run(
             "--no-use-kit",
         ]
         out(f"  $ {shlex.join(cmd)}")
-        rc = subprocess.call(cmd)
+        # Capture stdout+stderr to a file so we can ship the tail back
+        # in the response when the validator crashes. Streaming the
+        # SSE run log from outside the Space is unreliable (gradio's
+        # queue eats stderr, the HF /logs/run endpoint replays
+        # inconsistently). This is the only way the GitHub-Actions-
+        # side caller can see WHY the validator failed.
+        log_path = out_dir / "validator.log"
+        with log_path.open("wb") as logf:
+            rc = subprocess.call(cmd, stdout=logf, stderr=subprocess.STDOUT)
+        log_tail = ""
+        if log_path.exists():
+            try:
+                log_tail = log_path.read_text(encoding="utf-8", errors="replace")
+                log_tail = "\n".join(log_tail.splitlines()[-100:])
+            except OSError:
+                pass
         results_path = out_dir / "results.json"
         if rc != 0 and not results_path.exists():
             return RunResult(
                 dataset=dataset, profile=profile, version=version,
                 status="error",
-                summary=f"validator exited {rc}; no results.json produced",
+                summary=(f"validator exited {rc}; no results.json produced\n\n"
+                        f"--- validator log tail ---\n{log_tail}"),
                 results_json={}, report_path=out_dir, pr_url=None,
             )
         results_json = json.loads(results_path.read_text(encoding="utf-8"))