page_service: enable batching in Rust & Python Tests + Python benchmarks (#9993)

This is the first step towards batching rollout.

Refs

- rollout plan: https://github.com/neondatabase/cloud/issues/20620
- task https://github.com/neondatabase/neon/issues/9377
- uber-epic: https://github.com/neondatabase/neon/issues/9376

libs/pageserver_api/src/config.rs

@@ -442,7 +442,14 @@ impl Default for ConfigToml {
             tenant_config: TenantConfigToml::default(),
             no_sync: None,
             wal_receiver_protocol: DEFAULT_WAL_RECEIVER_PROTOCOL,
-            page_service_pipelining: PageServicePipeliningConfig::Serial,
+            page_service_pipelining: if !cfg!(test) {
+                PageServicePipeliningConfig::Serial
+            } else {
+                PageServicePipeliningConfig::Pipelined(PageServicePipeliningConfigPipelined {
+                    max_batch_size: NonZeroUsize::new(32).unwrap(),
+                    execution: PageServiceProtocolPipelinedExecutionStrategy::ConcurrentFutures,
+                })
+            },
         }
     }
 }

test_runner/fixtures/neon_fixtures.py

@@ -1095,6 +1095,17 @@ class NeonEnv:
                 # the pageserver taking a long time to start up due to syncfs flushing other tests' data
                 "no_sync": True,
             }
+
+            # Batching (https://github.com/neondatabase/neon/issues/9377):
+            # enable batching by default in tests and benchmarks.
+            # Compat tests are exempt because old versions fail to parse the new config.
+            if not config.compatibility_neon_binpath:
+                ps_cfg["page_service_pipelining"] = {
+                    "mode": "pipelined",
+                    "execution": "concurrent-futures",
+                    "max_batch_size": 32,
+                }
+
             if self.pageserver_virtual_file_io_engine is not None:
                 ps_cfg["virtual_file_io_engine"] = self.pageserver_virtual_file_io_engine
             if config.pageserver_default_tenant_config_compaction_algorithm is not None: