Fix/qa defects df002 df011 (#9)

## Summary

What changed and why?

## Change Type

- [ ] Cowork feature
- [ ] Bug fix
- [ ] Core AI contribution
- [ ] Test / hardening
- [ ] Performance
- [ ] Documentation

## Related Work

Cowork Task:

Core Repo: http://34.143.229.138/gitea-admin/fsg-ai-core-assets

Core AI Issue:

Core Task:

Related PR:

## Scope

What is intentionally included?

What is intentionally NOT included?

## Validation

- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual verification
- [ ] Regression check

Commands / evidence:

## Security Impact

Permission / credential / network / customer data impact:

## Compatibility

- [ ] No breaking change
- [ ] Breaking change documented

## Reviewer Notes

Anything Cowork reviewers should pay attention to.

---------

Co-authored-by: thanhnv <thanhnv.ip@gmail.com>
Co-authored-by: Vu Dam Tuan <vudt15@fpt.com>
Reviewed-on: #9

tests/test_monitoring_page_size.py

@@ -0,0 +1,70 @@
+"""DF-006 — the "Số dòng/trang" (rows per page) control: EventTable's
+page-size state (presentation/monitoring/shared/event_table.py) and its
+QComboBox wiring in build_filter_scaffold (.../shared/filter_scaffold.py).
+No dedicated test existed for this before — the design called for a
+user-visible/choosable item-per-page control, and this exercises it end to
+end (combo selection -> EventTable actually re-trimming its rows)."""
+from __future__ import annotations
+
+import os
+
+os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
+
+import pytest
+
+QApplication = pytest.importorskip("PySide6.QtWidgets").QApplication
+QWidget = pytest.importorskip("PySide6.QtWidgets").QWidget
+
+from cowork_local.presentation.monitoring.shared.event_table import (
+    PAGE_SIZE_OPTIONS, EventTable,
+)
+from cowork_local.presentation.monitoring.shared.filter_scaffold import build_filter_scaffold
+
+
+@pytest.fixture(scope="module")
+def qapp():
+    app = QApplication.instance() or QApplication([])
+    yield app
+
+
+def _events(n: int):
+    return [{"ts": f"2026-09-0{i % 9 + 1}T00:00:0{i % 9}", "kind": "tool_call",
+              "name": f"e{i}", "ok": True, "detail": ""} for i in range(n)]
+
+
+def test_default_page_size_matches_old_max_rows(qapp) -> None:
+    table = EventTable()
+    assert table.page_size() == 300
+    table.set_events(_events(500))
+    assert table.rowCount() == 300
+
+
+def test_set_page_size_retrims_without_reloading(qapp) -> None:
+    table = EventTable()
+    table.set_events(_events(500))
+    table.set_page_size(50)
+    assert table.page_size() == 50
+    assert table.rowCount() == 50
+
+
+def test_page_size_combo_is_only_added_when_requested(qapp) -> None:
+    page = QWidget()
+    table = EventTable()
+    parts = build_filter_scaffold(page, table, on_refresh=lambda: None, with_page_size=False)
+    assert "page_size_combo" not in parts
+
+
+def test_page_size_combo_changes_the_table(qapp) -> None:
+    page = QWidget()
+    table = EventTable()
+    table.set_events(_events(500))
+    parts = build_filter_scaffold(page, table, on_refresh=lambda: None, with_page_size=True)
+    combo = parts["page_size_combo"]
+    assert combo.count() == len(PAGE_SIZE_OPTIONS)
+    assert combo.currentData() == 300   # matches EventTable's current page_size
+
+    idx = PAGE_SIZE_OPTIONS.index(50)
+    combo.setCurrentIndex(idx)
+
+    assert table.page_size() == 50
+    assert table.rowCount() == 50