tests: pin disjoint /change concurrency at HTTP level

Closes the cubic acceptance-criteria gap (❌ "Integration test: two
/change requests targeting different (table_key, branch) execute
concurrently end-to-end"). The bench harness measures the throughput
side; this test is the regression sentinel that catches a future
change which accidentally re-introduces graph-wide serialization on
the disjoint path.

Spawns 4 concurrent /change inserts on node:Person and 4 on
node:Company. All 8 must return 200, and the post-test row counts
on each table must reflect every insert.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/omnigraph-server/tests/server.rs

@@ -2557,6 +2557,124 @@ async fn concurrent_branch_create_from_distinct_parents_does_not_corrupt_coordin
     );
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+async fn change_disjoint_table_concurrency_succeeds_at_http_level() {
+    // HTTP-level pin for MR-686's disjoint-table promise: concurrent /change
+    // requests touching different node types must coexist without admission
+    // rejection or publisher-CAS conflict. The bench harness measures
+    // throughput; this test is the regression sentinel that catches a
+    // future change which accidentally re-introduces graph-wide
+    // serialization on the disjoint path.
+    //
+    // Setup: test.jsonl seeds 4 Persons + 2 Companies. Spawn N=4 concurrent
+    // /change inserts on `node:Person` and N=4 concurrent inserts on
+    // `node:Company`. All 8 must return 200, and the post-test row counts
+    // must reflect every insert.
+    const PERSON_QUERY: &str = r#"
+query insert_p($name: String, $age: I32) {
+    insert Person { name: $name, age: $age }
+}
+"#;
+    const COMPANY_QUERY: &str = r#"
+query insert_c($name: String) {
+    insert Company { name: $name }
+}
+"#;
+    const SEED_PERSONS: u64 = 4;
+    const SEED_COMPANIES: u64 = 2;
+    const PER_TYPE: usize = 4;
+
+    let temp = init_loaded_repo().await;
+    let repo = repo_path(temp.path());
+    let state = AppState::open(repo.to_string_lossy().to_string())
+        .await
+        .unwrap();
+    let app = build_app(state);
+
+    let mut handles = Vec::with_capacity(PER_TYPE * 2);
+    for i in 0..PER_TYPE {
+        let app_p = app.clone();
+        handles.push(tokio::spawn(async move {
+            let body = serde_json::to_vec(&ChangeRequest {
+                query_source: PERSON_QUERY.to_string(),
+                query_name: Some("insert_p".to_string()),
+                params: Some(json!({ "name": format!("p-{i}"), "age": i as i32 })),
+                branch: Some("main".to_string()),
+            })
+            .unwrap();
+            let req = Request::builder()
+                .uri("/change")
+                .method(Method::POST)
+                .header("content-type", "application/json")
+                .body(Body::from(body))
+                .unwrap();
+            app_p.oneshot(req).await.unwrap().status()
+        }));
+        let app_c = app.clone();
+        handles.push(tokio::spawn(async move {
+            let body = serde_json::to_vec(&ChangeRequest {
+                query_source: COMPANY_QUERY.to_string(),
+                query_name: Some("insert_c".to_string()),
+                params: Some(json!({ "name": format!("c-{i}") })),
+                branch: Some("main".to_string()),
+            })
+            .unwrap();
+            let req = Request::builder()
+                .uri("/change")
+                .method(Method::POST)
+                .header("content-type", "application/json")
+                .body(Body::from(body))
+                .unwrap();
+            app_c.oneshot(req).await.unwrap().status()
+        }));
+    }
+
+    let mut statuses = Vec::with_capacity(PER_TYPE * 2);
+    for h in handles {
+        statuses.push(h.await.unwrap());
+    }
+
+    let bad: Vec<_> = statuses
+        .iter()
+        .enumerate()
+        .filter(|(_, s)| **s != StatusCode::OK)
+        .collect();
+    assert!(
+        bad.is_empty(),
+        "expected every disjoint /change insert to return 200, got non-200 for: {:?}",
+        bad,
+    );
+
+    // Verify both tables landed every insert.
+    let (status, body) = json_response(
+        &app,
+        Request::builder()
+            .uri("/snapshot?branch=main")
+            .method(Method::GET)
+            .body(Body::empty())
+            .unwrap(),
+    )
+    .await;
+    assert_eq!(status, StatusCode::OK);
+    let lookup_count = |table_key: &str| -> u64 {
+        body["tables"]
+            .as_array()
+            .and_then(|tables| tables.iter().find(|t| t["table_key"].as_str() == Some(table_key)))
+            .and_then(|t| t["row_count"].as_u64())
+            .unwrap_or_else(|| panic!("snapshot missing {}", table_key))
+    };
+    assert_eq!(
+        lookup_count("node:Person"),
+        SEED_PERSONS + PER_TYPE as u64,
+        "Person row count after concurrent inserts",
+    );
+    assert_eq!(
+        lookup_count("node:Company"),
+        SEED_COMPANIES + PER_TYPE as u64,
+        "Company row count after concurrent inserts",
+    );
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 4)]
 #[serial]
 async fn ingest_per_actor_admission_cap_returns_429() {