test: regression coverage for checkpoint dedup and inf/nan coercion

Covers the two bugs salvaged from PR #15161:

- test_batch_runner_checkpoint: TestFinalCheckpointNoDuplicates asserts
  the final aggregated completed_prompts list has no duplicate indices,
  and keeps a sanity anchor test documenting the pre-fix pattern so a
  future refactor that re-introduces it is caught immediately.

- test_model_tools: TestCoerceNumberInfNan asserts _coerce_number
  returns the original string for inf/-inf/nan/Infinity inputs and that
  the result round-trips through strict (allow_nan=False) json.dumps.

tests/test_batch_runner_checkpoint.py

@@ -186,3 +186,67 @@ class TestBatchWorkerResumeBehavior:
         assert result["discarded_no_reasoning"] == 1
         assert result["completed_prompts"] == [0]
         assert not batch_file.exists() or batch_file.read_text() == ""
+
+
+class TestFinalCheckpointNoDuplicates:
+    """Regression: the final checkpoint must not contain duplicate prompt
+    indices.
+
+    Before PR #15161, `run()` populated `completed_prompts_set` incrementally
+    as each batch completed, then at the end built `all_completed_prompts =
+    list(completed_prompts_set)` AND extended it again with every batch's
+    `completed_prompts` — double-counting every index.
+    """
+
+    def _simulate_final_aggregation_fixed(self, batch_results):
+        """Mirror the fixed code path in batch_runner.run()."""
+        completed_prompts_set = set()
+        for result in batch_results:
+            completed_prompts_set.update(result.get("completed_prompts", []))
+        # This is what the fixed code now writes to the checkpoint:
+        return sorted(completed_prompts_set)
+
+    def test_no_duplicates_in_final_list(self):
+        batch_results = [
+            {"completed_prompts": [0, 1, 2]},
+            {"completed_prompts": [3, 4]},
+            {"completed_prompts": [5]},
+        ]
+        final = self._simulate_final_aggregation_fixed(batch_results)
+        assert final == [0, 1, 2, 3, 4, 5]
+        assert len(final) == len(set(final))  # no duplicates
+
+    def test_persisted_checkpoint_has_unique_prompts(self, runner):
+        """Write what run()'s fixed aggregation produces to disk; the file
+        must load back with no duplicate indices."""
+        batch_results = [
+            {"completed_prompts": [0, 1]},
+            {"completed_prompts": [2, 3]},
+        ]
+        final = self._simulate_final_aggregation_fixed(batch_results)
+        runner._save_checkpoint({
+            "run_name": runner.run_name,
+            "completed_prompts": final,
+            "batch_stats": {},
+        })
+        loaded = json.loads(runner.checkpoint_file.read_text())
+        cp = loaded["completed_prompts"]
+        assert cp == sorted(set(cp))
+        assert len(cp) == 4
+
+    def test_old_buggy_pattern_would_have_duplicates(self):
+        """Document the bug this PR fixes: the old code shape produced
+        duplicates.  Kept as a sanity anchor so a future refactor that
+        re-introduces the pattern is immediately visible."""
+        completed_prompts_set = set()
+        results = []
+        for batch in ({"completed_prompts": [0, 1, 2]},
+                      {"completed_prompts": [3, 4]}):
+            completed_prompts_set.update(batch["completed_prompts"])
+            results.append(batch)
+        # Buggy aggregation (pre-fix):
+        buggy = list(completed_prompts_set)
+        for br in results:
+            buggy.extend(br.get("completed_prompts", []))
+        # Every index appears twice
+        assert len(buggy) == 2 * len(set(buggy))