Gate unified exec TTY support behind a feature flag (#42718)

## What changed

- Add the stable, default-on `unified_exec_tty` feature.
- When disabled, omit `tty` from the `exec_command` tool schema and reject calls that explicitly request a TTY.
- Continue to support commands that omit `tty` or set it to `false`.

## Testing

- Add unified exec coverage for the disabled tool schema, explicit TTY rejection, and non-TTY command execution.

GitOrigin-RevId: c5934fe9db1423170b891c7ecd0188792cd03f42

codex-rs/core/config.schema.json

@@ -1020,6 +1020,9 @@
             "unified_exec": {
               "type": "boolean"
             },
+            "unified_exec_tty": {
+              "type": "boolean"
+            },
             "unified_exec_zsh_fork": {
               "type": "boolean"
             },
@@ -6378,6 +6381,9 @@
         "unified_exec": {
           "type": "boolean"
         },
+        "unified_exec_tty": {
+          "type": "boolean"
+        },
         "unified_exec_zsh_fork": {
           "type": "boolean"
         },