feat(langsmith): support internal service URL overrides

[Philippe (Pckool)]

Summary

Adds opt-in internal service URL overrides for LangSmith's core configuration and frontend reverse-proxy routes.

Why

The chart currently embeds Kubernetes service FQDNs in core environment variables and NGINX upstreams. This is normally correct, but deployments with custom DNS, service meshes, or alternate internal routing need to direct these calls elsewhere.

Changes

• Add internalServiceUrls, an empty-by-default map of internal endpoint overrides.
• Route core ConfigMap endpoints, frontend NGINX upstreams, agent-feature API routes, and the dependent agent-gateway/host-backend environment variables through shared helpers.
• Preserve each existing Kubernetes service DNS URL when no override is supplied.
• Document the values and add unit coverage for default and override behavior.

Validation

• helm lint charts/langsmith -f charts/langsmith/ci/lightweight-config-values.yaml
• helm unittest charts/langsmith (63 tests)
• Rendered with custom URLs and confirmed no frontend proxy_pass retains a Kubernetes service FQDN.
• Compared the default render with upstream main; outputs are byte-identical.