project_settings: Fix default settings values for DiagnosticsSettings (#30686)

Follow-up to #30565

This PR fixes the default settings values for the `DiagnosticsSettings`.
The issue here was that due to the `#[derive(Default)]`, `button` would
be false by default, which unintentionally hid the diagnostics button by
default. The `#[serde(default = `default_true`)]` would only apply iff
the diagnostics key was already present in the user's settings. Thus, if
you have

```json
{
    "diagnostics": {...}
}
```

in your settings, the button would show (given it was not disabled).
However, if the key was not present, the button was not shown: Due to
the derived default for the entire struct, the value would be false.

This PR fixes this by implementing the default instead and moving the
`#[serde(default)]` up to the level of the struct.
I also did the same for the inline diagnostics settings, which already
had a default impl and thus only needed the serde default on the struct
instead of on all the struct fields.

Lastly, I simplified the title bar settings, since the serde attributes
previously had no effect anyway (deserialization happened in the
`TitlebarSettingsContent`, so these attributes had no effect) and we can
remove the `TitlebarSettingsContent` as well as the attributes if we
implement a proper default implementation instead.

Release Notes:

- Fixed the diagnostics status bar button being hidden by default.

crates/title_bar/src/title_bar_settings.rs

@@ -2,50 +2,48 @@ use db::anyhow;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsSources};
-use util::serde::default_true;
 
-#[derive(Deserialize, Debug, Clone, Copy, PartialEq)]
+#[derive(Copy, Clone, Serialize, Deserialize, JsonSchema, Debug)]
+#[serde(default)]
 pub struct TitleBarSettings {
-    #[serde(default)]
-    pub show_branch_icon: bool,
-    #[serde(default = "default_true")]
-    pub show_branch_name: bool,
-    #[serde(default = "default_true")]
-    pub show_project_items: bool,
-    #[serde(default = "default_true")]
-    pub show_onboarding_banner: bool,
-    #[serde(default = "default_true")]
-    pub show_user_picture: bool,
-}
-
-#[derive(Clone, Default, Serialize, Deserialize, JsonSchema, Debug)]
-pub struct TitleBarSettingsContent {
     /// Whether to show the branch icon beside branch switcher in the title bar.
     ///
     /// Default: false
-    pub show_branch_icon: Option<bool>,
+    pub show_branch_icon: bool,
     /// Whether to show onboarding banners in the title bar.
     ///
     /// Default: true
-    pub show_onboarding_banner: Option<bool>,
+    pub show_onboarding_banner: bool,
     /// Whether to show user avatar in the title bar.
     ///
     /// Default: true
-    pub show_user_picture: Option<bool>,
+    pub show_user_picture: bool,
     /// Whether to show the branch name button in the titlebar.
     ///
     /// Default: true
-    pub show_branch_name: Option<bool>,
+    pub show_branch_name: bool,
     /// Whether to show the project host and name in the titlebar.
     ///
     /// Default: true
-    pub show_project_items: Option<bool>,
+    pub show_project_items: bool,
+}
+
+impl Default for TitleBarSettings {
+    fn default() -> Self {
+        Self {
+            show_branch_icon: false,
+            show_onboarding_banner: true,
+            show_user_picture: true,
+            show_branch_name: true,
+            show_project_items: true,
+        }
+    }
 }
 
 impl Settings for TitleBarSettings {
     const KEY: Option<&'static str> = Some("title_bar");
 
-    type FileContent = TitleBarSettingsContent;
+    type FileContent = Self;
 
     fn load(sources: SettingsSources<Self::FileContent>, _: &mut gpui::App) -> anyhow::Result<Self>
     where