Add Cursor support to setup import (#33426)

## What changed

- Detect and import supported Cursor settings, sandbox permissions, MCP servers,
  project instructions, hooks, agents, commands, plugins, and recent chat
  sessions.
- Update the `/import` flow to check Claude Code and Cursor, prompting for a
  source when both have importable data.
- Add `migrationSource` to the app-server detect and import requests so each
  operation uses the selected adapter, while retaining `source` for import
  attribution and backwards compatibility.

## Testing

- Add coverage for Cursor config conversion, hooks, plugins, session discovery
  and conversion, app-server imports, and TUI source selection.

GitOrigin-RevId: a65a1511b6c1c2f9e3e0cfe4f816bb435807282e

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -435,6 +435,7 @@ fn external_agent_config_import_params_accept_legacy_plugin_details() {
                 }),
             }],
             source: None,
+            migration_source: None,
         }
     );
 }