agent: Add setting to control edit card expanded state (#34040)

This PR adds the `expand_edit_card` setting, which controls whether edit
cards in the agent panel are expanded, thus showing or not the full diff
of a given file's AI-driven change. I personally prefer to have these
cards collapsed by default as I am mostly reviewing diffs using either
the review multibuffer or the diffs within the file's buffer itself.
Didn't want to change the default behavior as that was intentionally
chosen, so here we are! :)

Open to feedback about the setting name; I've iterated between a few
options and don't necessarily feel like the current one is the best.

Release Notes:

- agent: Added a setting to control whether edit cards are expanded in
the agent panel, thus showing or hiding the full diff of a file's
changes.

crates/agent_settings/src/agent_settings.rs

@@ -67,6 +67,7 @@ pub struct AgentSettings {
     pub model_parameters: Vec<LanguageModelParameters>,
     pub preferred_completion_mode: CompletionMode,
     pub enable_feedback: bool,
+    pub expand_edit_card: bool,
 }
 
 impl AgentSettings {
@@ -291,6 +292,10 @@ pub struct AgentSettingsContent {
     ///
     /// Default: true
     enable_feedback: Option<bool>,
+    /// Whether to have edit cards in the agent panel expanded, showing a preview of the full diff.
+    ///
+    /// Default: true
+    expand_edit_card: Option<bool>,
 }
 
 #[derive(Clone, Copy, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Default)]
@@ -441,6 +446,7 @@ impl Settings for AgentSettings {
                 value.preferred_completion_mode,
             );
             merge(&mut settings.enable_feedback, value.enable_feedback);
+            merge(&mut settings.expand_edit_card, value.expand_edit_card);
 
             settings
                 .model_parameters