fix: enable programmatic config when environment variables are unset

[YangJonghun]

Which problem is this PR solving?

• Currently, when OTEL_NODE_ENABLED_INSTRUMENTATIONS is not set, users cannot enable default-excluded instrumentations (like fs) through programmatic configuration.
• Fixes the issue where getNodeAutoInstrumentations({"@opentelemetry/instrumentation-fs": {enabled: true}}) didn't work when environment variables were not configured.

Short description of the changes

• Establish clear configuration precedence: Environment variables > programmatic config
• Fix fs instrumentation enabling: When env vars are unset, allow programmatic config to override default exclusions
• Follow documented precedence: OTEL_NODE_DISABLED_INSTRUMENTATIONS overrides OTEL_NODE_ENABLED_INSTRUMENTATIONS when both are set, ensuring disabled instrumentations take absolute priority

Configuration Priority (after changes):

1. OTEL_NODE_DISABLED_INSTRUMENTATIONS - absolute priority
2. OTEL_NODE_ENABLED_INSTRUMENTATIONS - when set, only enable listed instrumentations
3. Programmatic config (userConfig.enabled) - when env vars are unset
4. Default behavior - exclude defaultExcludedInstrumentations unless explicitly enabled