Rerun sizing_pass when reopening popup (#8315)

* Closes <https://github.com/emilk/egui/issues/8115>
* [x] I have followed the instructions in the PR template

## Summary

- Recalculate a menu popup's cached `Area` size when it reopens.
- Preserve cached sizing for continuously open menus and leave tooltips
and general popups unchanged.
- Add a headless regression test covering a wider item added while the
menu is closed.

## Root cause

`Area` keeps its cached size after a menu closes. When that menu
reopened with wider content, the
cached width constrained the new item and caused it to wrap instead of
allowing the popup to grow.

The fix requests the same invisible sizing pass used for a first-open
`Area` whenever a menu was not
open during the previous frame.

## User impact

Menus now expand to fit newly added wider items after reopening.
Existing wrapping, explicit-width,
alignment, screen-constraining, and continuously open menu behavior
remain unchanged.

## Validation

- `cargo test -p egui_kittest --test menu`
- `cargo check -p egui`
- `cargo fmt --all -- --check`

crates/egui/src/containers/popup.rs

@@ -571,6 +571,7 @@ impl<'a> Popup<'a> {
             .fixed_pos(anchor)
             .sense(sense)
             .layout(layout)
+            .sizing_pass(!was_open_last_frame)
             .info(info.unwrap_or_else(|| {
                 UiStackInfo::new(kind.into()).with_tag_value(
                     MenuConfig::MENU_CONFIG_TAG,