keep a precise failure precise, and state one redirect policy

Two problems of the same shape: information about *why* something failed being
replaced by something vaguer.

get_dataset_geojson wrapped the JSON parse, the metadata read, the CRS
resolution and the canonicalisation in one try and reported all of it as
"Stored dataset is not valid JSON" with a 500. An operator whose dataset had an
unusable CRS was sent to inspect a file that parses perfectly well, and the
canonicaliser's own AppError — with its code and its status — never reached
them. Only the parse is now inside that handler; everything after it keeps the
error it raised, and a genuine bug becomes a distinct 500 rather than a
mislabelled client error. A guard finds the same shape elsewhere: catching
Exception around a call into another component and relabelling what it
reported. Wrapping one's own private helper stays legitimate and the guard
says so.

The redirect policy was split without anyone saying so. Two acquisition
services rejected every redirect through a hand-rolled opener, while eight
allowed a same-origin one through the shared guard — and only the latter
checked where the response came from. Both live in the guard now, and the
strict path uses the rejecting handler rather than the guard's after-the-fact
check: objecting to response.url means urllib already opened the connection and
read the body, which for a metadata endpoint is the whole attack. That was a
weakening I introduced in this same commit's first draft.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app/services/dataset_service.py

@@ -2906,8 +2906,19 @@ class DatasetService:
             raise AppError(code="DATASET_FILE_MISSING", message="Stored file missing", status_code=404)
 
         raw = load_dataset_text(dataset.storage_path)
+        # Only the parse can be "not valid JSON". Everything after it fails for
+        # its own reasons and must say so, or an operator is sent to inspect a
+        # file that parses perfectly well.
         try:
             payload = json.loads(raw)
+        except Exception as exc:
+            raise AppError(
+                code="INVALID_GEOJSON",
+                message="Stored dataset is not valid JSON",
+                status_code=500,
+            ) from exc
+
+        try:
             metadata_value = getattr(dataset, "metadata_json", None)
             metadata = metadata_value if isinstance(metadata_value, dict) else {}
             provenance_value = getattr(dataset, "provenance_metadata", None)
@@ -2933,8 +2944,17 @@ class DatasetService:
                 )
             )
             return VectorFeatureService.canonicalize_geojson_payload(payload, source_crs=str(source_crs))
+        except AppError:
+            # The canonicaliser's diagnosis is more precise than anything this
+            # layer could substitute for it.
+            raise
         except Exception as exc:
-            raise AppError(code="INVALID_GEOJSON", message="Stored dataset is not valid JSON", status_code=500) from exc
+            raise AppError(
+                code="DATASET_GEOJSON_UNREADABLE",
+                message="The stored dataset could not be read as canonical GeoJSON",
+                details={"dataset_id": str(dataset.id), "error_type": type(exc).__name__},
+                status_code=500,
+            ) from exc
 
     @staticmethod
     def inspect_vector_dataset(db: Session, dataset_id: UUID) -> dict[str, Any]: