Refactor/extract shared k8s module

[gCyrille]

Summary

This PR extracts shared XO/Kubernetes logic into the new xenorchestra-k8s-common module and upgrades dependencies to address stdlib vulnerabilities.

Changes

Refactor: delegate shared XO/k8s logic to xenorchestra-k8s-common

Removes duplicated code that has been consolidated into the shared xenorchestra-k8s-common library:

• Deleted pkg/provider/provider.go and tests — ProviderName, GetProviderID now come from xok8s
• Deleted pkg/xenorchestra/client.go and tests — XoClient and cloud config logic moved to xok8s
• Deleted pkg/xenorchestra/cloud_config.go and tests — cloud config parsing delegated to xok8s
• Deleted pkg/xenorchestra/xo_labels.go — XO label constants now from xok8s
• Updated pkg/xenorchestra/instances.go, cloud.go, and node label sync helpers to use the shared module

Build: dependency upgrades

• Upgraded Xen Orchestra Golang SDK to v1.13.0
• Upgraded Go toolchain to 1.25.8 to resolve stdlib security vulnerabilities
• Pinned github.com/vatesfr/xenorchestra-k8s-common to v0.1.0
• Upgraded k8s.io/* dependencies

Testing

• Existing unit tests updated
• No behavioral changes — this is a pure structural refactor

Related

• xenorchestra-k8s-common module: https://github.com/vatesfr/xenorchestra-k8s-common