HTTPRoute rules helper drops zero weight and backendRef filters

[aslafy-z]

Follow-up to #603. While reviewing the HTTPRoute backendRef port fix, two pre-existing defects were confirmed in the same application.httpRoute.rules helper block. They are out of scope for the port PR and should be fixed once it is merged.

1. weight: 0 is silently dropped

The renderer guards weight with a truthiness check ({{- if .weight }}), so an explicit weight: 0 is omitted from the manifest. Per the Gateway API, an omitted weight defaults to 1, so a backend the operator drained with weight: 0 keeps receiving traffic.

Reproduce:

httpRoute:
  enabled: true
  parentRefs:
    - name: my-gateway
  hostnames:
    - example.com
  rules:
    - backendRefs:
        - name: blue
          port: 80
          weight: 0
        - name: green
          port: 80
          weight: 100

Rendered output contains no weight field on blue. Fix with a presence check ({{- if not (kindIs "invalid" .weight) }}), the same idiom #603 uses for port.

Related: .weight | int silently coerces non-integer values (e.g. "10%") to 0 and the Gateway API range 0..1000000 is not validated. Applying the same validate-then-render pattern used for port would fail loudly instead.

2. Per-backendRef filters are silently discarded

The backendRef renderer is a field whitelist (name, port, weight, namespace, kind, group), so the HTTPBackendRef.filters field passes values.schema.json, renders successfully, and never reaches the cluster:

rules:
  - backendRefs:
      - name: example-service
        port: 80
        filters:
          - type: RequestHeaderModifier
            requestHeaderModifier:
              add:
                - name: x-canary
                  value: "true"

The rendered backendRef contains only name and port. Either add a filters passthrough (filters: {{ .filters | toYaml | nindent 6 }}) or render the whole ref via toYaml with coerced port/weight so future spec fields pass through as well. Rule-level matches/filters/timeouts already use the toYaml passthrough pattern.

Both behaviors were confirmed with live helm template renders against the chart.