fix cert rotation for standalone

[hidalgopl]

What issue type does this pull request address? (keep at least one, remove the others)
/kind bugfix

What does this pull request do? Which issues does it resolve? (use resolves #<issue_number> if possible)
resolves ENG-8077

Please provide a short message that should be published in the vcluster release notes
Fixed an issue where vcluster cert rotate was failing for vCluster Standalone

What else do we need to know?
Command was failing because we try to read namespace from the serviceaccount namespace file which doesn't exist for standalone. And the namespace isn't required for standalone certs, so we can set it to default.

---

Note

Sets NAMESPACE=default during standalone cert rotation to prevent failures when namespace is absent.

• Cert Rotation (standalone):
  • In cmd/vcluster/cmd/certs/rotate.go, after parsing standalone config, sets env var NAMESPACE to default if unset, with debug log on failure.
  • No changes to non-standalone path; continues using config.ParseConfig and existing rotation flow.

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[johannesfrey]

LGTM