Function resolveSignalEndpoint

Resolve the full OTLP endpoint URL for a given signal.

Priority (highest first):

1. OTEL_EXPORTER_OTLP_{SIGNAL}_ENDPOINT env var (full URL)
2. OTEL_EXPORTER_OTLP_ENDPOINT env var + /v1/{signal} suffix
3. config.{signal}ExporterEndpoint (full URL)
4. config.exporterEndpoint + /v1/{signal} suffix
5. undefined → signal is disabled