Improved error handling for kubeconfig errors

[EronWright]

Proposed changes

This PR tightens the kubeconfig loading logic of the provider.

1. Errors are not suppressed for the non-default provider (see Handle invalid kubeconfig context #960). The rationale is to stabilize the diff when the inputs are unprocessable, without breaking the invoke use-case.
2. Use a non-interactive loader for the kubeconfig, since the provider is a non-interactive server (see original PR). The rationale is to be consistent with the (newer) Helm code.
3. When the kubeconfig property is path-like, don't try to parse it as content. This improves the error message.

TODO: fix test: TestSkipUpdateUnreachableFlag

Related issues (optional)

[github-actions]

Does the PR have any schema changes?

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

[codecov-commenter]

Codecov Report

Merging #2599 (5be418f) into eronwright/issue-2486 (c52f572) will decrease coverage by 0.22%.
The diff coverage is 0.00%.

❗ Current head 5be418f differs from pull request most recent head a60ebd4. Consider uploading reports for the commit a60ebd4 to get more accurate results

@@                    Coverage Diff                    @@
##           eronwright/issue-2486    #2599      +/-   ##
=========================================================
- Coverage                  18.58%   18.37%   -0.22%     
=========================================================
  Files                         47       47              
  Lines                       9609     9610       +1     
=========================================================
- Hits                        1786     1766      -20     
- Misses                      7719     7745      +26     
+ Partials                     104       99       -5     

Files	Coverage Δ
provider/pkg/provider/util.go	59.25% <0.00%> (ø)
provider/pkg/provider/provider.go	7.26% <0.00%> (-0.01%)	⬇️

... and 2 files with indirect coverage changes

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[EronWright]

Sorry for any confusion about this PR, it was merged into a parent feature branch, see this PR: #2598