[codex] Track plugin install and import telemetry failures (#28731)

## Summary
- Track plugin install failures through the unified
`codex_plugin_install_failed` event for local installs, remote install
preflight failures, bundle failures, and remote catalog/backend
failures.
- Send classified `error_type` values in plugin install failure
analytics instead of raw error strings.
- Stop sending raw external-agent import errors in analytics while
preserving raw failure details in app-facing import
notifications/history.
- Keep raw plugin/migration diagnostics in `tracing::warn!` logs.
- Keep remote failure plugin names as the existing local placeholder
(`unknown`) and remove the extra telemetry plugin-name override.
- Change `ExternalAgentConfigImportParams.source` from a generated enum
to `string | null`, with legacy `claudeCode` / `claudeCowork` inputs
normalized to existing analytics values.

## Testing

codex-rs/tui/src/app_server_session.rs

@@ -391,7 +391,10 @@ impl AppServerSession {
             .client
             .request_typed(ClientRequest::ExternalAgentConfigImport {
                 request_id,
-                params: ExternalAgentConfigImportParams { migration_items },
+                params: ExternalAgentConfigImportParams {
+                    migration_items,
+                    source: None,
+                },
             })
             .await
             .wrap_err("externalAgentConfig/import failed during Claude Code import");