fix(helpers): render zero weight and backendRef filters in HTTPRoute

[aslafy-z]

Fixes #613. Follow-up to #603, fixing the two defects in the application.httpRoute.rules helper.

What changed

weight: 0 is now rendered. The renderer guarded weight with a truthiness check ({{- if .weight }}), so an explicit weight: 0 was omitted from the manifest. Since the Gateway API defaults an omitted weight to 1, a backend drained with weight: 0 kept receiving traffic. The guard is now a presence check, with the same empty-templated-value handling as port.

Per-backendRef filters are passed through. The backendRef renderer was a field whitelist (name, port, weight, namespace, kind, group), so HTTPBackendRef.filters validated against the schema but never reached the manifest. A toYaml passthrough now renders it, matching the existing rule-level matches/filters/timeouts handling.

Validation

• Three new helm-unittest cases: explicit weight: 0, empty templated weight treated as unset, and per-backendRef filters passthrough.
• Full suite passes (398 tests, 32 suites) and helm lint is clean.
• Confirmed the issue's repro with a live helm template render: weight: 0 and the RequestHeaderModifier filter now appear on the rendered backendRef.