fix(cli): handle kubeconfig servers without explicit ports

[immanuwell]

What issue type does this pull request address?
/kind bugfix

What does this pull request do? Which issues does it resolve?
resolves #3536

Fixes vcluster connect when the kubeconfig server has no explicit port, like https://example.com. Before this, the CLI split on : and bailed with unexpected server in kubeconfig. Bit of a footgun.

Repro:

1. Docker unavailable, so connect uses port-forward.
2. Kubeconfig server is https://example.com (no :443).
3. Run vcluster connect.

Now it parses the server as a URL, keeps explicit ports, and uses implicit https/http ports when needed.

Please provide a short message that should be published in the vcluster release notes
Fixed vcluster connect for kubeconfig server URLs without explicit ports.

What else do we need to know?
Tests: go test ./pkg/cli -run 'TestPortForwardServer|TestExchangeContextName', go test ./pkg/cli ./pkg/util/portforward, go test ./pkg/...

E2E Tests

none

[seanschneeweiss]

Same error message for both failure cases might be on purpose, right? Could be made more explicit by making the error text a const.

Suggested change

	return "", fmt.Errorf("unexpected server in kubeconfig: %s", parsed.String())
	return "", fmt.Errorf("unexpected server scheme in kubeconfig: %s", parsed.String())