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

tests/test_background_logic/src/main.rs

@@ -56,7 +56,8 @@ fn viewport_info(ctx: &egui::Context) -> String {
         ];
         for (name, value) in flags {
             if let Some(value) = value {
-                s += &format!(" {name}={value}");
+                use std::fmt::Write as _;
+                write!(s, " {name}={value}").ok();
             }
         }
         s