storage controller: fixes to secondary location handling (#7169)

Stacks on:
- https://github.com/neondatabase/neon/pull/7165

Fixes while working on background optimization of scheduling after a
split:
- When a tenant has secondary locations, we weren't detaching the parent
shards' secondary locations when doing a split
- When a reconciler detaches a location, it was feeding back a
locationconf with `Detached` mode in its `observed` object, whereas it
should omit that location. This could cause the background reconcile
task to keep kicking off no-op reconcilers forever (harmless but
annoying).
- During shard split, we were scheduling secondary locations for the
child shards, but no reconcile was run for these until the next time the
background reconcile task ran. Creating these ASAP is useful, because
they'll be used shortly after a shard split as the destination locations
for migrating the new shards to different nodes.

test_runner/fixtures/neon_fixtures.py

@@ -2150,6 +2150,18 @@ class NeonStorageController(MetricsGetter):
         shards: list[dict[str, Any]] = body["shards"]
         return shards
 
+    def tenant_describe(self, tenant_id: TenantId):
+        """
+        :return: list of {"shard_id": "", "node_id": int, "listen_pg_addr": str, "listen_pg_port": int, "listen_http_addr: str, "listen_http_port: int}
+        """
+        response = self.request(
+            "GET",
+            f"{self.env.storage_controller_api}/control/v1/tenant/{tenant_id}",
+            headers=self.headers(TokenScope.ADMIN),
+        )
+        response.raise_for_status()
+        return response.json()
+
     def tenant_shard_split(
         self, tenant_id: TenantId, shard_count: int, shard_stripe_size: Optional[int] = None
     ) -> list[TenantShardId]: