fix(host_agent): an ingest writes the newest five samples, so a full backlog can land (#5149)

Measured on dev (f802c9f): every dropped ingest carried samples=21,
metric_rows≈4,200 and never finished the INSERT flush inside the agent's
10 s ("completed: parse=0.1s"). The rollback left the agent's ring buffer
full, so each later post was the same size and failed the same way: no
host's metrics have landed since.

An ingest now keeps the newest MAX_SAMPLES_PER_INGEST (5) samples by ts,
logs how many it left out, and returns samples_dropped. The older samples
are the ones the ring buffer would have overwritten anyway. Why the
database writes so few rows a second is not yet known; the slow-ingest
line now measures it on a post that finishes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/plugins/host_agent/test_ingest_timing.py

@@ -56,3 +56,20 @@ async def test_a_dropped_ingest_says_how_far_it_got(app, monkeypatch, caplog):
         with caplog.at_level(logging.WARNING):
             phases.cancelled("abcd1234")
     assert "dropped after 10.1s for token=abcd1234… (completed: parse=0.2s) samples=21 metric_rows=840" in caplog.text
+
+
+def _s(minute):
+    return {"ts": f"2026-10-06T12:{minute:02d}:00+00:00", "cpu_pct": 1.0}
+
+
+def test_a_full_backlog_is_cut_to_its_newest_samples():
+    # Sent oldest first (the agent's buffer, then the new sample) or not.
+    backlog = [_s(m) for m in range(21)]
+    kept, dropped = routes.newest_samples(list(reversed(backlog)), 5)
+    assert [s["ts"] for s in kept] == [_s(m)["ts"] for m in range(16, 21)]
+    assert dropped == 16
+
+
+def test_a_small_post_is_left_alone():
+    posted = [_s(3), _s(1)]
+    assert routes.newest_samples(posted, 5) == (posted, 0)