fix(harness): correct truncated signalling for sql preview offset>0

When offset>0 the wrapper added an outer LIMIT N, so run_controlled's
_inject_limit bailed (a LIMIT IS present) and truncated was always False —
AGGrid could never detect more rows. Fix: for offset>0 probe with LIMIT (N+1)
OFFSET M, then compute truncated = len(rows) > N in do_run and slice back to N.
offset==0 path unchanged (delegates to extracted _run_transport helper). JSON
still reports the user's requested limit N and correct truncated. Adds 3 tests
exercising the real do_run offset>0 path (N+1 -> truncated True, N -> False,
offset==0 verbatim). 222/222 passing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

harness/tests/test_sql_preview_json.py

@@ -38,6 +38,75 @@ def test_inject_limit_offset_default_offset_zero():
     assert "OFFSET" not in out
 
 
+def test_do_run_offset_truncated_when_extra_row(monkeypatch):
+    """Real do_run offset>0 path: runner returns N+1 rows → truncated True, exactly N returned.
+
+    Stubs the transport layer (_run_transport) so do_run's own offset logic
+    (probe_limit=N+1, compute truncated, slice to N) is exercised for real.
+    """
+    from types import SimpleNamespace
+
+    from tht.cli import sql_cmd
+    from tht.execute import ExecResult
+
+    captured = {}
+
+    def fake_transport(cfg, sql, *, limit):
+        captured["sql"] = sql
+        captured["limit"] = limit
+        # runner sliced to probe_limit (N+1=3): it found the sentinel extra row.
+        return ExecResult(columns=["a"], rows=[(1,), (2,), (3,)], execution_ms=5, truncated=False)
+
+    monkeypatch.setattr(sql_cmd, "_run_transport", fake_transport)
+    result = sql_cmd.do_run(SimpleNamespace(), "SELECT a FROM t", limit=2, offset=10)
+
+    # do_run must have probed for N+1 rows and wrapped with OFFSET.
+    assert captured["limit"] == 3
+    assert "OFFSET 10" in captured["sql"] and "LIMIT 3" in captured["sql"]
+    # extra row detected → truncated True, sliced back to exactly N=2.
+    assert result.truncated is True
+    assert result.rows == [(1,), (2,)]
+
+
+def test_do_run_offset_not_truncated_when_exactly_n(monkeypatch):
+    """Real do_run offset>0 path: runner returns exactly N rows → truncated False."""
+    from types import SimpleNamespace
+
+    from tht.cli import sql_cmd
+    from tht.execute import ExecResult
+
+    def fake_transport(cfg, sql, *, limit):
+        return ExecResult(columns=["a"], rows=[(1,), (2,)], execution_ms=5, truncated=False)
+
+    monkeypatch.setattr(sql_cmd, "_run_transport", fake_transport)
+    result = sql_cmd.do_run(SimpleNamespace(), "SELECT a FROM t", limit=2, offset=10)
+
+    assert result.truncated is False
+    assert result.rows == [(1,), (2,)]
+
+
+def test_do_run_offset_zero_path_unchanged(monkeypatch):
+    """offset == 0 bypasses the wrapper entirely: SQL and limit reach the runner verbatim."""
+    from types import SimpleNamespace
+
+    from tht.cli import sql_cmd
+    from tht.execute import ExecResult
+
+    captured = {}
+
+    def fake_transport(cfg, sql, *, limit):
+        captured["sql"] = sql
+        captured["limit"] = limit
+        return ExecResult(columns=["a"], rows=[(1,)], execution_ms=5, truncated=False)
+
+    monkeypatch.setattr(sql_cmd, "_run_transport", fake_transport)
+    sql_cmd.do_run(SimpleNamespace(), "SELECT a FROM t", limit=2, offset=0)
+
+    # no wrapper applied: original SQL and original limit passed straight through.
+    assert captured["sql"] == "SELECT a FROM t"
+    assert captured["limit"] == 2
+
+
 def test_preview_json_pure_stdout(monkeypatch, tmp_path, capsys):
     from types import SimpleNamespace