acp: Support session modes (e.g. CC plan mode) (#37632)

Adds support for [ACP session
modes](https://github.com/zed-industries/agent-client-protocol/pull/67)
enabling plan and other permission modes in CC:


https://github.com/user-attachments/assets/dea18d82-4da6-465e-983b-02b77c6dcf15


Release Notes:

- Claude Code: Add support for plan mode, and all other permission modes

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/project/src/agent_server_store.rs

@@ -191,7 +191,10 @@ impl AgentServerStore {
                 fs: fs.clone(),
                 node_runtime: node_runtime.clone(),
                 project_environment: project_environment.clone(),
-                custom_command: new_settings.claude.clone().map(|settings| settings.command),
+                custom_command: new_settings
+                    .claude
+                    .clone()
+                    .and_then(|settings| settings.custom_command()),
             }),
         );
         self.external_agents
@@ -997,7 +1000,7 @@ pub const CLAUDE_CODE_NAME: &'static str = "claude";
 #[settings_key(key = "agent_servers")]
 pub struct AllAgentServersSettings {
     pub gemini: Option<BuiltinAgentServerSettings>,
-    pub claude: Option<CustomAgentServerSettings>,
+    pub claude: Option<BuiltinAgentServerSettings>,
 
     /// Custom agent servers configured by the user
     #[serde(flatten)]
@@ -1027,6 +1030,12 @@ pub struct BuiltinAgentServerSettings {
     ///
     /// Default: true
     pub ignore_system_version: Option<bool>,
+    /// The default mode to use for this agent.
+    ///
+    /// Note: Not only all agents support modes.
+    ///
+    /// Default: None
+    pub default_mode: Option<String>,
 }
 
 impl BuiltinAgentServerSettings {
@@ -1054,6 +1063,12 @@ impl From<AgentServerCommand> for BuiltinAgentServerSettings {
 pub struct CustomAgentServerSettings {
     #[serde(flatten)]
     pub command: AgentServerCommand,
+    /// The default mode to use for this agent.
+    ///
+    /// Note: Not only all agents support modes.
+    ///
+    /// Default: None
+    pub default_mode: Option<String>,
 }
 
 impl settings::Settings for AllAgentServersSettings {