Add MCP tool handler support to the hooks engine (#38705)

## What changed

- Discover synchronous `mcp_tool` hook handlers and invoke their configured MCP server and tool through a supplied executor.
- Expand nested hook-event placeholders in MCP tool inputs while preserving JSON types, and process tool output through the existing hook output contract.
- Represent hook details as handler-specific metadata in `hooks/list`, including MCP server and tool fields, and show those details in the TUI hooks browser.
- Skip unsupported `SessionEnd` MCP hooks and runtimes without MCP invocation support with startup warnings.

## Testing

- Cover argument expansion, missing placeholders, MCP invocation and hook decisions, discovery warnings, `hooks/list` metadata, and TUI rendering.

GitOrigin-RevId: 295b845471fe92bd7ad7cd272fbcd2c3713912e0

codex-rs/app-server-protocol/src/protocol/v2/hook.rs

@@ -23,7 +23,7 @@ v2_enum_from_core!(
 
 v2_enum_from_core!(
     pub enum HookHandlerType from CoreHookHandlerType {
-        Command, Prompt, Agent
+        Command, McpTool, Prompt, Agent
     }
 );
 

codex-rs/app-server-protocol/src/protocol/v2/plugin.rs

@@ -1,7 +1,5 @@
 use super::AppSummary;
 use super::HookEventName;
-use super::HookExecutionMode;
-use super::HookHandlerType;
 use super::HookSource;
 use super::HookTrustStatus;
 use crate::JsonSchema;
@@ -518,17 +516,32 @@ pub struct HooksListEntry {
     pub errors: Vec<HookErrorInfo>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(tag = "handlerType", rename_all = "camelCase")]
+#[ts(tag = "handlerType", export_to = "v2/")]
+pub enum HookHandlerMetadata {
+    Command {
+        command: String,
+        #[serde(default)]
+        r#async: bool,
+    },
+    McpTool {
+        server: String,
+        tool: String,
+    },
+    Prompt {},
+    Agent {},
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct HookMetadata {
     pub key: String,
     pub event_name: HookEventName,
-    pub handler_type: HookHandlerType,
-    #[serde(default)]
-    pub execution_mode: HookExecutionMode,
+    #[serde(flatten)]
+    pub handler: HookHandlerMetadata,
     pub matcher: Option<String>,
-    pub command: Option<String>,
     pub timeout_sec: u64,
     pub status_message: Option<String>,
     /// Configured `additionalContext` spill threshold.

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -3396,6 +3396,45 @@ fn user_input_into_core_preserves_media_fields() {
     );
 }
 
+#[test]
+fn hook_handler_metadata_only_exposes_async_for_commands() {
+    assert_eq!(
+        serde_json::to_value(HookHandlerMetadata::Command {
+            command: "echo hello".to_string(),
+            r#async: true,
+        })
+        .unwrap(),
+        json!({
+            "handlerType": "command",
+            "command": "echo hello",
+            "async": true,
+        }),
+    );
+    assert_eq!(
+        serde_json::from_value::<HookHandlerMetadata>(json!({
+            "handlerType": "command",
+            "command": "echo hello",
+        }))
+        .unwrap(),
+        HookHandlerMetadata::Command {
+            command: "echo hello".to_string(),
+            r#async: false,
+        },
+    );
+    assert_eq!(
+        serde_json::to_value(HookHandlerMetadata::McpTool {
+            server: "security".to_string(),
+            tool: "scan".to_string(),
+        })
+        .unwrap(),
+        json!({
+            "handlerType": "mcpTool",
+            "server": "security",
+            "tool": "scan",
+        }),
+    );
+}
+
 #[test]
 fn skills_list_params_serialization_uses_force_reload() {
     assert_eq!(