Include session titles in external agent import history (#35870)

## What changed

- Preserve the original title when importing an external-agent session and return it with successful session entries from `externalAgentConfig/import/readHistories`.
- Add dedicated history-record success types so externally completed imports can optionally supply a session title without changing the live import result type.

## Testing

- Verify imported session titles are returned by the app server and persisted when session import ledger entries are created or refreshed.

GitOrigin-RevId: e896688a5763c238f1e1dc8b4672a7aa7bc1ed4c

codex-rs/external-agent-migration/src/model.rs

@@ -122,13 +122,19 @@ impl ExternalAgentConfigImportItemResult {
         self.raw_errors.push(raw_error);
     }
 
-    pub fn record_success(&mut self, source: Option<String>, target: Option<String>) {
+    pub fn record_success(
+        &mut self,
+        source: Option<String>,
+        target: Option<String>,
+        title: Option<String>,
+    ) {
         self.success_count = self.success_count.saturating_add(1);
         self.successes.push(ExternalAgentConfigImportSuccess {
             item_type: self.item_type,
             cwd: self.cwd.clone(),
             source,
             target,
+            title,
         });
     }
 }
@@ -139,6 +145,7 @@ pub struct ExternalAgentConfigImportSuccess {
     pub cwd: Option<PathBuf>,
     pub source: Option<String>,
     pub target: Option<String>,
+    pub title: Option<String>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]