[app-server] feat: v2 Turn APIs (#6216)

Implements:
```
turn/start
turn/interrupt
```

along with their integration tests. These are relatively light wrappers
around the existing core logic, and changes to core logic are minimal.

However, an improvement made for developer ergonomics:
- `turn/start` replaces both `SendUserMessage` (no turn overrides) and
`SendUserTurn` (can override model, approval policy, etc.)

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -133,6 +133,18 @@ client_request_definitions! {
         params: v2::ThreadCompactParams,
         response: v2::ThreadCompactResponse,
     },
+    #[serde(rename = "turn/start")]
+    #[ts(rename = "turn/start")]
+    TurnStart {
+        params: v2::TurnStartParams,
+        response: v2::TurnStartResponse,
+    },
+    #[serde(rename = "turn/interrupt")]
+    #[ts(rename = "turn/interrupt")]
+    TurnInterrupt {
+        params: v2::TurnInterruptParams,
+        response: v2::TurnInterruptResponse,
+    },
 
     #[serde(rename = "model/list")]
     #[ts(rename = "model/list")]