OpenTelemetry header helper: prefetching and failure warnings

What

• For OpenTelemetry (OTel, a standard for exporting telemetry data) exporters using HTTP-based protocols (http/json or http/protobuf), a user-configured otelHeadersHelper is now prefetched once per process to prime its cache, instead of being fetched lazily on the first request.
• If otelHeadersHelper fails, Claude Code now shows a dismissible warning notification (visible for 30 seconds) telling you that telemetry is not being exported and to check /status for details.

Why

Prefetching avoids a delay on the first telemetry request, and the new warning makes it clear when your custom header helper is broken instead of silently dropping telemetry exports.