[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/app-server-protocol/src/protocol/v2/tests.rs

@@ -376,6 +376,7 @@ fn external_agent_config_import_params_accept_legacy_plugin_details() {
                     ..Default::default()
                 }),
             }],
+            source: None,
         }
     );
 }