fix(cli): support legacy use_linux_sandbox_bwrap flag (#14473)

## Summary
- restore `use_linux_sandbox_bwrap` as a removed feature key so older
`--enable` callers parse again
- keep it as a no-op by leaving runtime behavior unchanged
- add regression coverage for the legacy `--enable` path

## Testing
- Not run (updated and pushed quickly)

codex-rs/core/config.schema.json

@@ -483,6 +483,9 @@
             "use_legacy_landlock": {
               "type": "boolean"
             },
+            "use_linux_sandbox_bwrap": {
+              "type": "boolean"
+            },
             "voice_transcription": {
               "type": "boolean"
             },
@@ -1988,6 +1991,9 @@
         "use_legacy_landlock": {
           "type": "boolean"
         },
+        "use_linux_sandbox_bwrap": {
+          "type": "boolean"
+        },
         "voice_transcription": {
           "type": "boolean"
         },