Report CLI as the external agent config import source (#33883)

## Why

The TUI import request used the selected migration source as the identifier for
the product initiating the import, conflating two distinct protocol fields.

## What changed

Set `source` to `cli` for imports initiated by the TUI while continuing to pass
the selected adapter through `migrationSource`.

GitOrigin-RevId: 7b00728464cb1d4e478ac4aa6b18f7254d2e5152

codex-rs/tui/src/app_server_session.rs

@@ -453,7 +453,7 @@ impl AppServerSession {
                 request_id,
                 params: ExternalAgentConfigImportParams {
                     migration_items,
-                    source: Some(migration_source.clone()),
+                    source: Some("cli".to_string()),
                     migration_source: Some(migration_source),
                 },
             })