fix(ci): use helm-charts develop branch for K8s E2E tests (#25118)

Instead of pulling the published chart from helm.vector.dev, clone
the helm-charts repo (develop branch) and use the local chart path.
This breaks the circular dependency between Vector and helm-charts
releases and catches chart incompatibilities before release.

- Add `helm_chart_repo()` helper with HELM_CHART_REPO env var override
- Update deploy-chart-test.sh to support local chart paths
- Clone helm-charts in CI and set HELM_CHART_REPO

Fixes: #25111

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>