Standardize shell execution on unified exec (#39757)

## What changed

- Remove the legacy `shell_command` handler and runtime, leaving `exec_command`
  and `write_stdin` as the shell execution tools.
- Treat legacy `shell_command` model metadata as `unified_exec`, and normalize
  legacy user opt-outs so they do not disable command execution. Managed feature
  requirements and `shell_tool` can still disable it.
- Preserve shell approvals, sandboxing, zsh-fork support, and output truncation
  through the unified execution path.

## Testing

- Cover legacy configuration and model-metadata compatibility.
- Exercise unified shell execution, approvals, truncation, and `apply_patch`
  serialization across the app-server and core test suites.

GitOrigin-RevId: 5c2fd6164fc3519cdae4944cb9db276b8467311c

sdk/typescript/tests/abort.test.ts

@@ -4,7 +4,7 @@ import {
   assistantMessage,
   responseCompleted,
   responseStarted,
-  shell_call as shellCall,
+  exec_command_call as execCommandCall,
   sse,
   SseResponseBody,
   startResponsesTestProxy,
@@ -13,7 +13,7 @@ import { createMockClient } from "./testCodex";
 
 function* infiniteShellCall(): Generator<SseResponseBody> {
   while (true) {
-    yield sse(responseStarted(), shellCall(), responseCompleted());
+    yield sse(responseStarted(), execCommandCall(), responseCompleted());
   }
 }
 

sdk/typescript/tests/responsesProxy.ts

@@ -180,16 +180,16 @@ export function assistantMessage(text: string, itemId: string = DEFAULT_MESSAGE_
   };
 }
 
-export function shell_call(): SseEvent {
+export function exec_command_call(): SseEvent {
   return {
     type: "response.output_item.done",
     item: {
       type: "function_call",
       call_id: `call_id${Math.random().toString(36).slice(2)}`,
-      name: "shell_command",
+      name: "exec_command",
       arguments: JSON.stringify({
-        command: "echo 'Hello, world!'",
-        timeout_ms: 100,
+        cmd: "echo 'Hello, world!'",
+        yield_time_ms: 100,
       }),
     },
   };