Fix TextEdit hint text not following horizontal_align/vertical_align (#8332)

## Summary

- Closes #8309
- [x] I have followed the instructions in the PR template

`TextEdit` hint text was always aligned to `Align2::LEFT_TOP`, ignoring
the alignment set via `TextEdit::horizontal_align` / `vertical_align`.
This caused the hint text, the cursor, and the typed text to disagree on
alignment: e.g. a centered `TextEdit` showed a left-aligned hint with a
centered cursor.

The hint text atoms now use the widget's `align`, so the hint matches
the input text alignment. The default `align` is still `LEFT_TOP`, so
multi line text edits (and the default styling) are unchanged.

### Root cause

In `crates/egui/src/widgets/text_edit/builder.rs`, the hint-text branch
hardcoded:

```rust
atoms.push_right(atom.atom_align(Align2::LEFT_TOP));
```

while the input-text branch used `.atom_align(self.align)`. The hint
path now uses `align` as well.

### Drive-by: silence `clippy::unnecessary_wraps` in
`egui_kittest::app_kind`

`AppKind::run` returns `Option<egui::Response>`. The `Option` wrap is
required when the `eframe` feature is enabled (the `Eframe` branch
returns `None`), but `clippy::unnecessary_wraps` fires when
`egui_kittest` is built standalone without the `eframe` feature (e.g.
`cargo clippy -p egui_kittest`). The workspace CI run doesn't hit it
because feature unification via `egui_demo_app` enables `eframe`, but
it's a real annoyance for anyone linting the crate on its own. Added a
scoped `#[cfg_attr(not(feature = "eframe"),
expect(clippy::unnecessary_wraps))]` with an explanatory comment.

## Test plan

- [x] Added `textedit_hint_text_should_follow_text_alignment` kittest
regression in `crates/egui_kittest/tests/regression_tests.rs`. It fails
before the fix (`hint_center_x=24.25` vs `edit_center_x=100`) and passes
after.
- [x] `cargo test -p egui`
- [x] `cargo test -p egui_kittest --all-features --test
regression_tests`
- [x] `cargo clippy -p egui_kittest --all-features --test
regression_tests -- -D warnings`
- [x] `RUSTFLAGS="-D warnings" cargo clippy -p egui_kittest --lib`
(pre-existing `unnecessary_wraps` now silenced)
- [x] `cargo clippy -p egui -- -D warnings`
- [x] `cargo fmt --check`

---------

Co-authored-by: Lucas Meurer <hi@lucasmerlin.me>

crates/egui/src/widgets/text_edit/builder.rs

@@ -622,9 +622,10 @@ impl TextEdit<'_> {
                         first = false;
                     }
 
-                    // The hint text should be shown left top instead of centered (important for
+                    // Align the hint text the same as the input text so the hint, the
-                    // multi line text edits)
+                    // cursor, and the typed text all share one alignment. The default
-                    atoms.push_right(atom.atom_align(Align2::LEFT_TOP));
+                    // `align` is `LEFT_TOP`, which keeps multi line text edits unchanged.
+                    atoms.push_right(atom.atom_align(align));
                 }
 
                 // Calculate the empty galley, so it can be read later. The available width is

crates/egui_kittest/src/app_kind.rs

@@ -23,6 +23,10 @@ pub(crate) enum AppKind<'a, State> {
 }
 
 impl<State> AppKind<'_, State> {
+    // The `Option` is needed when the `eframe` feature is enabled, because the
+    // `Eframe` variant has no `egui::Response` to return. Without `eframe` the
+    // wrap is unnecessary, so we silence `clippy::unnecessary_wraps` for that case.
+    #[cfg_attr(not(feature = "eframe"), expect(clippy::unnecessary_wraps))]
     pub fn run(
         &mut self,
         ui: &mut egui::Ui,

crates/egui_kittest/tests/regression_tests.rs

@@ -713,3 +713,49 @@ fn collapsing_panel_must_not_grow_enclosing_window() {
         );
     }
 }
+
+/// The hint text of a `TextEdit` should follow the same alignment as the input
+/// text, instead of always being left-top aligned.
+///
+/// Regression test for <https://github.com/emilk/egui/issues/8309>.
+#[test]
+pub fn textedit_hint_text_should_follow_text_alignment() {
+    let mut input = String::new();
+
+    let mut harness = Harness::builder()
+        .with_size(Vec2::new(200.0, 40.0))
+        .build_ui(|ui| {
+            ui.add(
+                egui::TextEdit::singleline(&mut input)
+                    .hint_text("Hint")
+                    .desired_width(200.0)
+                    .horizontal_align(egui::Align::Center),
+            );
+        });
+    harness.run();
+
+    let text_edit = harness.get_by_role(accesskit::Role::TextInput);
+    let edit_rect = text_edit.rect();
+
+    // Find the hint text shape (the only text shape while the input is empty).
+    let hint_shape = harness
+        .output()
+        .shapes
+        .iter()
+        .find_map(|clipped| {
+            let egui::epaint::Shape::Text(text_shape) = &clipped.shape else {
+                return None;
+            };
+            (text_shape.galley.text() == "Hint").then_some(text_shape)
+        })
+        .expect("hint text shape should be painted");
+
+    let hint_center_x = hint_shape.pos.x + hint_shape.galley.size().x / 2.0;
+    let edit_center_x = edit_rect.center().x;
+
+    assert!(
+        (hint_center_x - edit_center_x).abs() < 1.0,
+        "hint text should be centered in the TextEdit: hint_center_x={hint_center_x}, \
+         edit_center_x={edit_center_x}, edit_rect={edit_rect:?}",
+    );
+}