Make external session detection limits configurable (#34449)

## What changed

- Add `maxSessionAgeDays` and `maxSessions` to the external-agent config detection request.
- Apply the requested limits when discovering sessions from supported migration sources.
- Preserve the existing defaults of a 30-day maximum age and 50 sessions when either option is omitted.

GitOrigin-RevId: 46376e94b64f1f61d8776e34d82cdd2cdeb36ca0

codex-rs/external-agent-migration/src/detect/mod.rs

@@ -378,9 +378,11 @@ impl ExternalAgentConfigService {
         }
 
         if scope.is_home() {
-            let sessions = self
-                .source
-                .recent_sessions(&self.external_agent_home, &self.codex_home)?;
+            let sessions = self.source.recent_sessions(
+                &self.external_agent_home,
+                &self.codex_home,
+                self.session_import_limits,
+            )?;
             if !sessions.is_empty() {
                 items.push(ExternalAgentConfigMigrationItem {
                     item_type: ExternalAgentConfigMigrationItemType::Sessions,