[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/src/message_processor.rs

@@ -515,7 +515,7 @@ impl MessageProcessor {
             outgoing.clone(),
             config_manager.clone(),
             thread_manager.clone(),
-            analytics_events_client,
+            analytics_events_client.clone(),
         );
         let external_agent_config_processor =
             ExternalAgentConfigRequestProcessor::new(ExternalAgentConfigRequestProcessorArgs {
@@ -525,6 +525,7 @@ impl MessageProcessor {
                 config_manager: config_manager.clone(),
                 config_processor: config_processor.clone(),
                 state_db,
+                analytics_events_client,
                 arg0_paths,
                 codex_home: config.codex_home.to_path_buf(),
             });