debugger: Update New Session Modal (#30018)

This PR simplifies the new session modal by flattening its three modes
and updating the UI to be less noisy. The new UI also defaults to the
Debug Scenario Picker, and allows users to save debug scenarios created
in the UI to the active worktree's .zed/debug.json file.


Release Notes:

- N/A

crates/task/src/debug_format.rs

@@ -193,22 +193,22 @@ pub struct DebugScenario {
     /// Name of the debug task
     pub label: SharedString,
     /// A task to run prior to spawning the debuggee.
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub build: Option<BuildTaskDefinition>,
     #[serde(flatten)]
     pub request: Option<DebugRequest>,
     /// Additional initialization arguments to be sent on DAP initialization
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub initialize_args: Option<serde_json::Value>,
     /// Optional TCP connection information
     ///
     /// If provided, this will be used to connect to the debug adapter instead of
     /// spawning a new process. This is useful for connecting to a debug adapter
     /// that is already running or is started by another process.
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub tcp_connection: Option<TcpArgumentsTemplate>,
     /// Whether to tell the debug adapter to stop on entry
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub stop_on_entry: Option<bool>,
 }