test(db): add abort-racing-send + rejectAllWaiters coverage; document notifyOneWaiter invariant

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

node/tests/db-watch.test.ts

@@ -107,4 +107,49 @@ describe("MailboxStore.waitForMessage", () => {
       expect([r1.message.body, r2.message.body]).toEqual(["m1", "m2"]);
     }
   });
+
+  it("abort racing send: message is either delivered or remains pending, never lost", async () => {
+    store.upsertMailbox("alice");
+    store.upsertMailbox("bob");
+
+    const ac = new AbortController();
+    const pending = store.waitForMessage("bob", 5000, ac.signal);
+
+    // Fire abort and send in the same tick — either order is valid as long as the message is never lost.
+    ac.abort();
+    store.send("alice", "bob", "racy");
+
+    const r = await pending;
+    if (r.kind === "aborted") {
+      // Message must still be in DB for the next caller.
+      expect(store.peek("bob").pending).toBe(1);
+    } else if (r.kind === "message") {
+      expect(r.message.body).toBe("racy");
+      expect(store.peek("bob").pending).toBe(0);
+    } else {
+      throw new Error(`unexpected kind: ${r.kind}`);
+    }
+  });
+
+  it("rejectAllWaiters resolves every pending waiter as aborted and empties the bucket map", async () => {
+    store.upsertMailbox("bob");
+    store.upsertMailbox("carol");
+
+    const ac1 = new AbortController();
+    const ac2 = new AbortController();
+    const ac3 = new AbortController();
+    const w1 = store.waitForMessage("bob", 5000, ac1.signal);
+    const w2 = store.waitForMessage("bob", 5000, ac2.signal);
+    const w3 = store.waitForMessage("carol", 5000, ac3.signal);
+
+    // Give all three a chance to register their waiters.
+    await new Promise((r) => setTimeout(r, 10));
+
+    store.rejectAllWaiters();
+
+    const [r1, r2, r3] = await Promise.all([w1, w2, w3]);
+    expect(r1.kind).toBe("aborted");
+    expect(r2.kind).toBe("aborted");
+    expect(r3.kind).toBe("aborted");
+  });
 });