Report detailed session import error types (#33863)

## What changed

- Add stable `sub_error_type` values for external-agent session import failures,
  including detection, preparation, configuration, thread storage, and ledger
  update errors.
- Preserve these values through import results and analytics while leaving
  unrelated plugin and memory errors unchanged.

## Testing

- Verify that a missing session reports `session_not_detected` in both the
  completed import result and its analytics event.

GitOrigin-RevId: cbf1f52df4bb1ac107442c4afa0b130fa32ad0a5

codex-rs/app-server/src/external_agent_migration/processor.rs

@@ -270,6 +270,7 @@ impl ExternalAgentConfigRequestProcessor {
                             record_import_error(
                                 &mut item_result,
                                 "plugin_import",
+                                /*sub_error_type*/ None,
                                 error.to_string(),
                                 /*source*/ None,
                             );
@@ -400,6 +401,7 @@ impl ExternalAgentConfigRequestProcessor {
                         record_import_error(
                             &mut item_result,
                             "session_missing",
+                            Some("session_not_detected"),
                             format!(
                                 "external agent session was not detected for import: {}",
                                 session.path.display()
@@ -412,6 +414,7 @@ impl ExternalAgentConfigRequestProcessor {
                         record_import_error(
                             &mut item_result,
                             "session_source_path",
+                            Some("failed_to_resolve_session_source_path"),
                             err.to_string(),
                             Some(session.path.display().to_string()),
                         );