Enable a few more clippy lints (#8064)

Enable these new clippy lints and fix all warnings:

* `format_push_string` — use `write!` instead of `s += &format!(…)` to
avoid extra allocations
* `ignored_unit_patterns` — use `()` instead of `_` when matching unit
* `missing_fields_in_debug` — ensure manual `Debug` impls account for
all fields
* `needless_raw_string_hashes` — remove unnecessary `r#` on string
literals
* `ref_option` — prefer `Option<&T>` over `&Option<T>` in function
signatures

crates/egui_kittest/src/snapshot.rs

@@ -563,7 +563,7 @@ pub fn image_snapshot_options(
     options: &SnapshotOptions,
 ) {
     match try_image_snapshot_options(current, name, options) {
-        Ok(_) => {}
+        Ok(()) => {}
         Err(err) => {
             panic!("{err}");
         }
@@ -582,7 +582,7 @@ pub fn image_snapshot_options(
 #[track_caller]
 pub fn image_snapshot(current: &image::RgbaImage, name: impl Into<String>) {
     match try_image_snapshot(current, name) {
-        Ok(_) => {}
+        Ok(()) => {}
         Err(err) => {
             panic!("{err}");
         }
@@ -884,14 +884,14 @@ impl Drop for SnapshotResults {
             #[expect(clippy::manual_assert)]
             if count >= 2 {
                 panic!(
-                    r#"
+                    "
 Multiple SnapshotResults were dropped without being handled.
 
 In order to allow consistent snapshot updates, all snapshot results within a test should be merged in a single SnapshotResults instance.
 Usually this is handled internally in a harness. If you have multiple harnesses, you can merge the results using `Harness::take_snapshot_results` and `SnapshotResults::extend`.
 
 The SnapshotResult was constructed at {}
-                    "#,
+                    ",
                     self.location
                 );
             }