Support chart.v3 previews when cluster is unreachable

[blampe]

We currently fail to render a preview for Chart V3 if the cluster is unreachable.

Instead of failing, we can emit a warning since Helm is still able to generate the template without the version set.

Alternatively, we could check k.clusterUnreachable as part of Invoke but we wouldn't be able to return a rich preview.

Added a failing E2E test.

Fixes #2985.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 29.94%. Comparing base (785d414) to head (a76337a).

❗ Current head a76337a differs from pull request most recent head 0a4e29d. Consider uploading reports for the commit 0a4e29d to get more accurate results

Files	Patch %	Lines
provider/pkg/provider/invoke_helm_template.go	0.00%	4 Missing ⚠️
provider/pkg/provider/provider.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2992      +/-   ##
==========================================
- Coverage   29.99%   29.94%   -0.06%     
==========================================
  Files          63       63              
  Lines        8338     8353      +15     
==========================================
  Hits         2501     2501              
- Misses       5615     5630      +15     
  Partials      222      222              

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