Fix/qa defects df002 df003 df006 (#12)

## 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: Vu Dam Tuan <vudt15@fpt.com>
Co-authored-by: Duy Le Huu <duylh19@fpt.com>
Reviewed-on: #12

tests/test_monitoring_page_size.py

@@ -1,9 +1,11 @@
-"""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)."""
+"""DF-006 — the "Số dòng/trang" (rows per page) control plus real prev/next
+pagination: EventTable's page-size/page-index state
+(presentation/monitoring/shared/event_table.py) and its QComboBox + pager
+button wiring in build_filter_scaffold (.../shared/filter_scaffold.py).
+
+Options are 5/10/20/50/100 with a next/prev pager, per the QA follow-up on
+DF-006 — the earlier fix only trimmed to a page size (dropping every row past
+it with no way back); this exercises the real paging end to end."""
 from __future__ import annotations
 
 import os
@@ -32,19 +34,62 @@ def _events(n: int):
               "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_page_size_options_are_5_10_20_50_100() -> None:
+    assert PAGE_SIZE_OPTIONS == (5, 10, 20, 50, 100)
 
 
-def test_set_page_size_retrims_without_reloading(qapp) -> None:
+def test_default_page_size(qapp) -> None:
     table = EventTable()
-    table.set_events(_events(500))
+    assert table.page_size() == 20
+    table.set_events(_events(45))
+    assert table.rowCount() == 20
+    assert table.page_count() == 3
+    assert table.current_page() == 0
+
+
+def test_set_page_size_resets_to_first_page(qapp) -> None:
+    table = EventTable()
+    table.set_events(_events(45))
+    table.next_page()
+    assert table.current_page() == 1
     table.set_page_size(50)
     assert table.page_size() == 50
-    assert table.rowCount() == 50
+    assert table.current_page() == 0
+    assert table.rowCount() == 45   # only 45 events total, fits in one page of 50
+
+
+def test_next_prev_page_navigate_without_dropping_rows(qapp) -> None:
+    table = EventTable()
+    table.set_events(_events(45))
+    table.set_page_size(20)
+    assert table.rowCount() == 20
+
+    table.next_page()
+    assert table.current_page() == 1
+    assert table.rowCount() == 20
+
+    table.next_page()
+    assert table.current_page() == 2
+    assert table.rowCount() == 5   # last page: remainder
+
+    table.next_page()   # already on last page — stays put
+    assert table.current_page() == 2
+
+    table.prev_page()
+    assert table.current_page() == 1
+    table.prev_page()
+    table.prev_page()   # already on first page — stays put
+    assert table.current_page() == 0
+
+
+def test_page_changed_signal_reports_current_and_total(qapp) -> None:
+    table = EventTable()
+    seen = []
+    table.page_changed.connect(lambda cur, total: seen.append((cur, total)))
+    table.set_events(_events(45))
+    table.set_page_size(20)
+    table.next_page()
+    assert seen[-1] == (1, 3)
 
 
 def test_page_size_combo_is_only_added_when_requested(qapp) -> None:
@@ -52,19 +97,44 @@ def test_page_size_combo_is_only_added_when_requested(qapp) -> None:
     table = EventTable()
     parts = build_filter_scaffold(page, table, on_refresh=lambda: None, with_page_size=False)
     assert "page_size_combo" not in parts
+    assert "page_prev_btn" not in parts
 
 
 def test_page_size_combo_changes_the_table(qapp) -> None:
     page = QWidget()
     table = EventTable()
-    table.set_events(_events(500))
+    table.set_events(_events(45))
     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
+    assert combo.currentData() == 20   # matches EventTable's current page_size
 
-    idx = PAGE_SIZE_OPTIONS.index(50)
+    idx = PAGE_SIZE_OPTIONS.index(10)
     combo.setCurrentIndex(idx)
 
-    assert table.page_size() == 50
-    assert table.rowCount() == 50
+    assert table.page_size() == 10
+    assert table.rowCount() == 10
+
+
+def test_pager_buttons_disable_at_bounds_and_indicator_updates(qapp) -> None:
+    page = QWidget()
+    table = EventTable()
+    table.set_events(_events(45))
+    parts = build_filter_scaffold(page, table, on_refresh=lambda: None, with_page_size=True)
+    table.set_page_size(20)
+    prev_btn, next_btn = parts["page_prev_btn"], parts["page_next_btn"]
+    indicator = parts["page_indicator_label"]
+
+    assert not prev_btn.isEnabled()
+    assert next_btn.isEnabled()
+    assert indicator.text() == "Trang 1/3"
+
+    next_btn.click()
+    assert prev_btn.isEnabled()
+    assert next_btn.isEnabled()
+    assert indicator.text() == "Trang 2/3"
+
+    next_btn.click()
+    assert prev_btn.isEnabled()
+    assert not next_btn.isEnabled()
+    assert indicator.text() == "Trang 3/3"