fix(observability): preserve engine span resource identity

[alsruf36]

Summary

• Resolve engine-originated OpenTelemetry resource identity from configured service metadata / OTEL environment variables while ignoring blank values.
• Attach normalized service.name, service.version, and optional service.namespace to the engine tracer provider resource.
• Forward provider resources through TeeSpanExporter so OTEL_EXPORTER_TYPE=both exports the same service identity to external OTLP collectors.

Test Plan

• cargo test -p iii --test otel_tracer_provider_resource_test
• cargo test -p iii --test otel_tracer_provider_both_resource_test
• cargo test -p iii --test otel_forwarder_resource_test
• cargo fmt --all --check

Notes

This PR is stacked on top of iii-hq#1618 (fix/1617-otlp-resource-forwarding). iii-hq#1618 fixes SDK-forwarded spans by preserving inbound OTLP resources; this PR fixes the separate engine-originated SdkTracerProvider export path for iii-hq#1619.

Live verification with OTEL_EXPORTER_TYPE=both confirmed SigNoz now receives the expected engine service identity instead of the collector/backend fallback <nil>.