Expose managed policy for browser settings imports (#39720)

## What changed

- Add `in_app_browser.allow_external_browser_settings_import` to managed requirements, preserving explicit Boolean values through layered composition while leaving an omitted value unset.
- Return the policy as `inAppBrowser.allowExternalBrowserSettingsImport` from `configRequirements/read` and include it in the generated protocol schemas.
- Keep the import policy independent from the in-app browser feature flag and agent Browser Use requirements.

## Testing

- Cover parsing, managed-layer precedence, invalid values, user and session override resistance, and app-server response serialization.

GitOrigin-RevId: efa2621d2b1cf503f1bee2505d9914cb4fb7221d

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -9064,6 +9064,16 @@
               }
             ]
           },
+          "inAppBrowser": {
+            "anyOf": [
+              {
+                "$ref": "#/definitions/v2/InAppBrowserRequirements"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
           "logDir": {
             "type": [
               "string",
@@ -12419,6 +12429,17 @@
           }
         ]
       },
+      "InAppBrowserRequirements": {
+        "properties": {
+          "allowExternalBrowserSettingsImport": {
+            "type": [
+              "boolean",
+              "null"
+            ]
+          }
+        },
+        "type": "object"
+      },
       "InputModality": {
         "description": "Canonical user-input modality tags advertised by a model.",
         "oneOf": [