fix(services): не балансировать сервис с одним IP

KPI и карточка показывают живой OK, а не гистерезис unknown; DNS по-прежнему ждёт повторные успехи.

Co-authored-by: Cursor <[email protected]>

apps/api/test/lb-reconcile.test.ts

@@ -158,6 +158,34 @@ describe("resolveDesiredAIps", () => {
     ).toEqual(["1.1.1.1"]);
   });
 
+  it("does not drain when the service has only one unique IP", () => {
+    expect(
+      resolveDesiredAIps(
+        weightedConfig,
+        [row("1.1.1.1", { health: "down" })],
+        ["1.1.1.1", "1.1.1.1"],
+        0,
+        ["1.1.1.1"],
+      ),
+    ).toEqual(["1.1.1.1", "1.1.1.1"]);
+  });
+
+  it("does not apply overlay when service pool is a single IP", () => {
+    const rows = [
+      row("1.1.1.1", { health: "up" }),
+      row("2.2.2.2", { health: "down" }),
+    ];
+    expect(
+      resolveDesiredAIps(
+        weightedConfig,
+        rows,
+        ["1.1.1.1", "2.2.2.2"],
+        0,
+        ["1.1.1.1"],
+      ),
+    ).toEqual(["1.1.1.1", "2.2.2.2"]);
+  });
+
   it("applies weighted overlay on a shared pool", () => {
     const rows = [
       row("1.1.1.1", { weight: 1, health: "up" }),
@@ -170,6 +198,18 @@ describe("resolveDesiredAIps", () => {
 });
 
 describe("shouldRecordFailoverDnsDiff", () => {
+  it("skips duplicate listings of the same IP", () => {
+    expect(
+      shouldRecordFailoverDnsDiff({
+        configuredIps: ["1.1.1.1", "1.1.1.1"],
+        lbMode: "failover",
+        added: [],
+        removed: ["1.1.1.1"],
+        downIps: new Set(["1.1.1.1"]),
+      }),
+    ).toBe(false);
+  });
+
   it("skips dedicated extra-FQDN", () => {
     expect(
       shouldRecordFailoverDnsDiff({