feat(charts): make HTTPRoute backendRefs optional per rule

[IxDay]

Summary

lago-rails and lago-front always appended the default service backendRefs to every rule in their HTTPRoute template. That blocked rules ending in a terminal filter (DirectResponse via ExtensionRef, RequestRedirect, etc.), since those must render without a backendRefs to be valid Gateway API.

The change uses hasKey on .backendRefs per rule:

• omitted → default service backend injected (back-compat with every existing values file)
• backendRefs: [] → no backendRefs rendered, for terminal-filter rules
• explicit list → passed through for custom or weighted backends

HTTPRouteRule.backendRefs is optional in the Gateway API spec, so this just stops the chart from overriding that optionality.

Motivation: lago-infrastructure needs to add a /metrics block rule (DirectResponse 404 via Envoy Gateway's HTTPRouteFilter) to the lago-api HTTPRoute in staging-eu. Since that route is templated by lago-rails, the chart had to grow a hook for terminal filters.

Test plan

• helm template charts/lago-rails -f <values> with backendRefs: [] on a rule renders the rule with no backendRefs: line
• same with backendRefs: [{...}] renders the user backends
• same with no backendRefs key renders the default service backend (back-compat)
• same three cases on charts/lago-front
• task render on the umbrella lago chart exits clean
• (deferred) task test:unit once untt is available locally