[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/analytics/src/events.rs

@@ -83,6 +83,9 @@ pub(crate) enum TrackEventRequest {
     PluginUninstalled(CodexPluginEventRequest),
     PluginEnabled(CodexPluginEventRequest),
     PluginDisabled(CodexPluginEventRequest),
+    PluginInstallFailed(CodexPluginInstallFailedEventRequest),
+    ExternalAgentConfigImportCompleted(CodexOnboardingExternalAgentImportCompleteEventRequest),
+    ExternalAgentConfigImportFailure(CodexOnboardingExternalAgentImportFailureEventRequest),
 }
 
 impl TrackEventRequest {
@@ -954,6 +957,53 @@ pub(crate) struct CodexPluginEventRequest {
     pub(crate) event_params: CodexPluginMetadata,
 }
 
+#[derive(Serialize)]
+pub(crate) struct CodexPluginInstallFailedMetadata {
+    #[serde(flatten)]
+    pub(crate) plugin: CodexPluginMetadata,
+    pub(crate) error_type: String,
+}
+
+#[derive(Serialize)]
+pub(crate) struct CodexPluginInstallFailedEventRequest {
+    pub(crate) event_type: &'static str,
+    pub(crate) event_params: CodexPluginInstallFailedMetadata,
+}
+
+#[derive(Serialize)]
+pub(crate) struct CodexOnboardingExternalAgentImportCompleteMetadata {
+    pub(crate) import_id: String,
+    pub(crate) source: String,
+    #[serde(rename = "type")]
+    pub(crate) item_type: String,
+    pub(crate) success_count: usize,
+    pub(crate) failed_count: usize,
+    pub(crate) product_client_id: Option<String>,
+}
+
+#[derive(Serialize)]
+pub(crate) struct CodexOnboardingExternalAgentImportCompleteEventRequest {
+    pub(crate) event_type: &'static str,
+    pub(crate) event_params: CodexOnboardingExternalAgentImportCompleteMetadata,
+}
+
+#[derive(Serialize)]
+pub(crate) struct CodexOnboardingExternalAgentImportFailureMetadata {
+    pub(crate) import_id: String,
+    pub(crate) source: String,
+    #[serde(rename = "type")]
+    pub(crate) item_type: String,
+    pub(crate) failure_stage: String,
+    pub(crate) error_type: String,
+    pub(crate) product_client_id: Option<String>,
+}
+
+#[derive(Serialize)]
+pub(crate) struct CodexOnboardingExternalAgentImportFailureEventRequest {
+    pub(crate) event_type: &'static str,
+    pub(crate) event_params: CodexOnboardingExternalAgentImportFailureMetadata,
+}
+
 #[derive(Serialize)]
 pub(crate) struct CodexPluginUsedEventRequest {
     pub(crate) event_type: &'static str,