[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/schema/json/codex_app_server_protocol.schemas.json

@@ -9119,7 +9119,7 @@
             ]
           },
           "includeHome": {
-            "description": "If true, include detection under the user's home (~/.claude, ~/.codex, etc.).",
+            "description": "If true, include detection under the user's home directory.",
             "type": "boolean"
           }
         },
@@ -9282,6 +9282,13 @@
               "$ref": "#/definitions/v2/ExternalAgentConfigMigrationItem"
             },
             "type": "array"
+          },
+          "source": {
+            "description": "Source product that produced the migration items. Missing means unspecified.",
+            "type": [
+              "string",
+              "null"
+            ]
           }
         },
         "required": [