test: fail prefix roundtrip when no traces are tested

benchmarks/prefix_data_generator/tests/test_roundtrip_hashes.py

@@ -97,6 +97,7 @@ def test_hash_roundtrip_direct(self):
             # Phase 2: Test roundtrip
             roundtrip_hasher = RollingHasher(block_size=DEFAULT_BLOCK_SIZE)
             mismatches = []
+            generation_failures = []
             tested = 0
 
             for i, trace in enumerate(traces):
@@ -113,6 +114,7 @@ def test_hash_roundtrip_direct(self):
                     )
                 except Exception as e:
                     print(f"  Trace {i}: Generation failed - {e}")
+                    generation_failures.append((i, str(e)))
                     continue
 
                 # Step 2: Re-encode text (simulates what server does)
@@ -142,6 +144,18 @@ def test_hash_roundtrip_direct(self):
 
             print(f"\nTested {tested} traces with hash_ids")
 
+            if tested == 0:
+                failure_details = ""
+                if generation_failures:
+                    preview = "; ".join(
+                        f"trace {i}: {error}" for i, error in generation_failures[:5]
+                    )
+                    failure_details = f" Generation failures: {preview}"
+                pytest.fail(
+                    "No traces were successfully tested for hash roundtrip."
+                    f"{failure_details}"
+                )
+
             # Report results
             if mismatches:
                 print(f"\nFound {len(mismatches)} hash mismatches:")