fix: command pipe execution by improving shell detection (#437)

## Description
This PR fixes Issue #421 where commands with pipes (e.g., `grep -R ...
-n | head -n 20`) were failing to execute properly after PR #391 was
merged.

## Changes
- Modified the `requiresShell` function to only enable shell mode when
the command is a single string containing shell operators
- Added logic to handle the case where shell operators are passed as
separate arguments
- Added comprehensive tests to verify the fix

## Root Cause
The issue was that the `requiresShell` function was detecting shell
operators like `|` even when they were passed as separate arguments,
which caused the command to be executed with `shell: true`
unnecessarily. This was causing syntax errors when running commands with
pipes.

## Testing
- Added unit tests to verify the fix
- Manually tested with real commands using pipes
- Ensured all existing tests pass

Fixes #421

codex-cli/src/utils/agent/exec.ts

@@ -14,10 +14,16 @@ import { parse } from "shell-quote";
 const DEFAULT_TIMEOUT_MS = 10_000; // 10 seconds
 
 function requiresShell(cmd: Array<string>): boolean {
-  return cmd.some((arg) => {
-    const tokens = parse(arg) as Array<ParseEntry>;
+  // If the command is a single string that contains shell operators,
+  // it needs to be run with shell: true
+  if (cmd.length === 1 && cmd[0] !== undefined) {
+    const tokens = parse(cmd[0]) as Array<ParseEntry>;
     return tokens.some((token) => typeof token === "object" && "op" in token);
-  });
+  }
+
+  // If the command is split into multiple arguments, we don't need shell: true
+  // even if one of the arguments is a shell operator like '|'
+  return false;
 }
 
 /**