server: drainer abandons only on ErrNotFound, not transient errors

GetSourceGroup errors in drainOne now gate on errors.Is(err,
store.ErrNotFound) before calling abandonPending, mirroring the
existing GetSchedule pattern. Transient errors (SQLITE_BUSY, context
cancellation) now log a warning and return without deleting the row.

Add regression test TestDrainPendingDropsRowsForGoneSourceGroup
confirming the ErrNotFound path still abandons correctly. Also add
a comment above the backoff-doubling loop explaining the progression.

internal/server/http/pending_drain_test.go

@@ -341,6 +341,62 @@ func TestDrainPendingDropsRowsForGoneSchedule(t *testing.T) {
 	}
 }
 
+// TestDrainPendingDropsRowsForGoneSourceGroup verifies that when a
+// source group is gone (ErrNotFound) the pending row is abandoned and
+// an audit entry is written. Transient-error paths (SQLITE_BUSY,
+// context cancellation) are not covered here because the real *Store
+// doesn't expose a fault-injection seam; the code-review check above
+// is the gate for that path.
+func TestDrainPendingDropsRowsForGoneSourceGroup(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	hostID, token := enrolHostForWS(t, srv, st, "gone-group-host")
+	_, sid := seedSchedAndGroup(t, st, hostID, 5)
+
+	// Use a source_group_id that never existed. pending_runs carries a
+	// FK to source_groups, so we must bypass FK enforcement for this
+	// insert. PRAGMA foreign_keys is connection-scoped and can only be
+	// changed outside a transaction; DB().Exec runs on an arbitrary
+	// pooled connection, so we pin it with a dedicated *sql.Conn.
+	fakeGroupID := ulid.Make().String()
+	pendingID := ulid.Make().String()
+	now := time.Now().UTC()
+	conn, err := st.DB().Conn(context.Background())
+	if err != nil {
+		t.Fatalf("db conn: %v", err)
+	}
+	defer conn.Close()
+	if _, err := conn.ExecContext(context.Background(), `PRAGMA foreign_keys = OFF`); err != nil {
+		t.Fatalf("fk off: %v", err)
+	}
+	if _, err := conn.ExecContext(context.Background(),
+		`INSERT INTO pending_runs (id, schedule_id, source_group_id, host_id, attempt, next_attempt_at, scheduled_at)
+		 VALUES (?, ?, ?, ?, 1, ?, ?)`,
+		pendingID, sid, fakeGroupID, hostID,
+		now.Add(-time.Second), now.Add(-time.Minute),
+	); err != nil {
+		t.Fatalf("insert pending: %v", err)
+	}
+	if _, err := conn.ExecContext(context.Background(), `PRAGMA foreign_keys = ON`); err != nil {
+		t.Fatalf("fk on: %v", err)
+	}
+
+	auditBefore := countAuditAction(t, st, "pending_run.abandoned")
+
+	c := agentDial(t, srv, ts, hostID, token)
+	sendHello(t, c, "gone-group-host")
+	_ = drainUntil(t, c, api.MsgScheduleSet)
+
+	srv.DrainPending(context.Background(), hostID)
+
+	if n := countPendingForHost(t, st, hostID); n != 0 {
+		t.Errorf("pending rows after source-group-gone abandon: got %d, want 0", n)
+	}
+	if d := countAuditAction(t, st, "pending_run.abandoned") - auditBefore; d != 1 {
+		t.Errorf("audit delta: got %d, want 1", d)
+	}
+}
+
 func TestDrainAllDueSkipsOfflineHosts(t *testing.T) {
 	t.Parallel()
 	srv, _, st := rawTestServer(t)