api/metadata: Fix issues detected with hypothesis

Running metadata tests with multiple hyothesis examples uncovered those
issues in api-1-raw-extrinsic-metadata-swhid Web API view:

  - RawExtrinsincMetaData only targetting core SWHIds must be provided as test inputs.

  - link-next URL for pagination was invalid.

  - next_page_token must be encoded before providing it to urlsafe_b64encode.

swh/web/api/views/metadata.py

@@ -145,14 +145,18 @@ def api_raw_extrinsic_metadata_swhid(request, target):
         "results": results,
         "headers": {},
     }
+
     if result_page.next_page_token is not None:
         response["headers"]["link-next"] = reverse(
-            "api-1-raw-extrinsic-metadata",
+            "api-1-raw-extrinsic-metadata-swhid",
+            url_args={"target": target},
             query_params=dict(
                 authority=authority_str,
                 after=after_str,
                 limit=limit_str,
-                page_token=base64.urlsafe_b64encode(result_page.next_page_token),
+                page_token=base64.urlsafe_b64encode(
+                    result_page.next_page_token.encode()
+                ),
             ),
             request=request,
         )

swh/web/tests/api/views/test_metadata.py

@@ -5,14 +5,34 @@
 
 import attr
 from hypothesis import given, strategies
+from hypothesis.strategies._internal.core import sampled_from
 import pytest
 
-from swh.model.hypothesis_strategies import raw_extrinsic_metadata
+from swh.model.hypothesis_strategies import (
+    raw_extrinsic_metadata as raw_extrinsic_metadata_orig,
+)
+from swh.model.hypothesis_strategies import sha1_git
+from swh.model.identifiers import ExtendedObjectType, ExtendedSWHID, ObjectType
 from swh.web.common.utils import reverse
 from swh.web.tests.api.views.utils import scroll_results
 from swh.web.tests.utils import check_api_get_responses, check_http_get_response
 
 
+# public Web API endpoint for raw extrinsic metadata does not support
+# extended SWHIDs so we ensure only core ones will be used in test inputs.
+@strategies.composite
+def raw_extrinsic_metadata(draw):
+    remd = draw(raw_extrinsic_metadata_orig())
+    remd = attr.evolve(
+        remd,
+        target=ExtendedSWHID(
+            object_type=ExtendedObjectType(draw(sampled_from(ObjectType)).value),
+            object_id=draw(sha1_git()),
+        ),
+    )
+    return attr.evolve(remd, id=remd.compute_hash())
+
+
 @given(raw_extrinsic_metadata())
 def test_api_raw_extrinsic_metadata(api_client, subtest, metadata):
     # ensure archive_data fixture will be reset between each hypothesis