test(host_agent): ring buffer and payload builder unit tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/plugins/host_agent/test_agent_ring_buffer.py

@@ -0,0 +1,24 @@
+from plugins.host_agent.agent import RingBuffer
+
+
+def test_ring_buffer_preserves_order():
+    rb = RingBuffer(maxlen=3)
+    rb.push(1); rb.push(2); rb.push(3)
+    assert list(rb.drain()) == [1, 2, 3]
+    assert len(rb) == 0
+
+
+def test_ring_buffer_drops_oldest_when_full():
+    rb = RingBuffer(maxlen=3)
+    for v in (1, 2, 3, 4, 5):
+        rb.push(v)
+    assert list(rb.drain()) == [3, 4, 5]
+
+
+def test_ring_buffer_drain_clears_and_is_atomic():
+    rb = RingBuffer(maxlen=5)
+    rb.push("a"); rb.push("b")
+    out = list(rb.drain())
+    rb.push("c")
+    assert out == ["a", "b"]
+    assert list(rb.drain()) == ["c"]