Expose connector candidates from imported sessions (#33414)

## What changed

- Correlate MCP server attribution in imported session records with connector
  metadata, and persist normalized connector names in the session import ledger.
- Extend `externalAgentConfig/import/readHistories` with connector candidates,
  including each connector's display name, imported-session count, and detection
  source.

## Testing

- Cover connector detection through the session import and history-read flow.
- Verify candidate aggregation uses the latest import for each source session.

GitOrigin-RevId: 7f1f7f596925b2e6efcd17afd4490148e1ac9ff1

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

@@ -756,6 +756,23 @@ pub struct ExternalAgentConfigImportHistory {
 #[ts(export_to = "v2/")]
 pub struct ExternalAgentConfigImportHistoriesReadResponse {
     pub data: Vec<ExternalAgentConfigImportHistory>,
+    pub connectors: Vec<ExternalAgentImportedConnectorCandidate>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub enum ExternalAgentImportedConnectorSource {
+    RemoteMcpServersConfig,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ExternalAgentImportedConnectorCandidate {
+    pub name: String,
+    pub session_count: u32,
+    pub source: ExternalAgentImportedConnectorSource,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]