Don't unarchive a timeline if its ancestor is archived (#8853)

If a timeline unarchival request comes in, give an error if the parent
timeline is archived. This prevents us from the situation of having an
archived timeline with children that are not archived.

Follow up of #8824

Part of #8088

---------

Co-authored-by: Joonas Koivunen <joonas@neon.tech>

test_runner/regress/test_timeline_archive.py

@@ -94,3 +94,29 @@ def test_timeline_archive(neon_simple_env: NeonEnv):
         timeline_id=parent_timeline_id,
         state=TimelineArchivalState.ARCHIVED,
     )
+
+    # Test that the leaf can't be unarchived
+    with pytest.raises(
+        PageserverApiException,
+        match="ancestor is archived",
+    ) as exc:
+        assert timeline_path.exists()
+
+        ps_http.timeline_archival_config(
+            tenant_id=env.initial_tenant,
+            timeline_id=leaf_timeline_id,
+            state=TimelineArchivalState.UNARCHIVED,
+        )
+
+    # Unarchive works for the leaf if the parent gets unarchived first
+    ps_http.timeline_archival_config(
+        tenant_id=env.initial_tenant,
+        timeline_id=parent_timeline_id,
+        state=TimelineArchivalState.UNARCHIVED,
+    )
+
+    ps_http.timeline_archival_config(
+        tenant_id=env.initial_tenant,
+        timeline_id=leaf_timeline_id,
+        state=TimelineArchivalState.UNARCHIVED,
+    )