[app-server] small fixes for JSON schema export and one-of types (#6614)

A partner is consuming our generated JSON schema bundle for app-server
and identified a few issues:
- not all polymorphic / one-of types have a type descriminator
- `"$ref": "#/definitions/v2/SandboxPolicy"` is missing
- "Option<>" is an invalid schema name, and also unnecessary

This PR:
- adds the type descriminator to the various types that are missing it
except for `SessionSource` and `SubAgentSource` because they are
serialized to disk (adding this would break backwards compat for
resume), and they should not be necessary to consume for an integration
with app-server.
- removes the special handling in `export.rs` of various types like
SandboxPolicy, which turned out to be unnecessary and incorrect
- filters out `Option<>` which was auto-generated for request params
that don't need a body

For context, we currently pull in wayyy more types than we need through
the `EventMsg` god object which we are **not** planning to expose in API
v2 (this is how I suspect `SessionSource` and `SubAgentSource` are being
pulled in). But until we have all the necessary v2 notifications in
place that will allow us to remove `EventMsg`, we will keep exporting it
for now.

codex-rs/tui/src/chatwidget/tests.rs

@@ -100,6 +100,37 @@ fn upgrade_event_payload_for_tests(mut payload: serde_json::Value) -> serde_json
                     serde_json::Value::String(aggregated),
                 );
             }
+        } else if ty == "patch_apply_begin"
+            && let Some(changes) = m.get_mut("changes").and_then(|v| v.as_object_mut())
+        {
+            for change in changes.values_mut() {
+                if change.get("type").is_some() {
+                    continue;
+                }
+                if let Some(change_obj) = change.as_object_mut()
+                    && change_obj.len() == 1
+                    && let Some((legacy_kind, legacy_value)) = change_obj
+                        .iter()
+                        .next()
+                        .map(|(k, v)| (k.clone(), v.clone()))
+                {
+                    change_obj.clear();
+                    change_obj.insert(
+                        "type".to_string(),
+                        serde_json::Value::String(legacy_kind.clone()),
+                    );
+                    match legacy_value {
+                        serde_json::Value::Object(payload) => {
+                            for (k, v) in payload {
+                                change_obj.insert(k, v);
+                            }
+                        }
+                        other => {
+                            change_obj.insert("content".to_string(), other);
+                        }
+                    }
+                }
+            }
         }
     }
     payload