feat: add default linux nodeSelector to listener pod

[gateixeira]

The listener is a Linux-only binary. In mixed-OS clusters (e.g. with Windows node pools), the listener pod can be scheduled on a Windows node where it fails with a platform mismatch.

Issue #2652 was closed by #2758 which gave users a workaround via listenerTemplate, but the default behavior still allows the listener to be scheduled on Windows nodes. This PR adds a safe default (kubernetes.io/os: linux) as a defensive approach so users don't need to explicitly configure listenerTemplate just to prevent the listener from landing on a non-Linux node.

Changes:

• Add default nodeSelector kubernetes.io/os: linux to listener pod spec
• Define LabelKeyKubernetesOS constant alongside existing k8s labels
• Make mergeListenerPodWithTemplate preserve the default when the listenerTemplate does not explicitly set a nodeSelector (nil check)
• Add unit tests for all nodeSelector merge scenarios

Refs: #2652

[Copilot]

Pull request overview

Adds a defensive default to ensure the scale set listener pod (a Linux-only binary) is scheduled onto Linux nodes in mixed-OS Kubernetes clusters, without requiring users to set a custom listenerTemplate.

Changes:

• Default the listener pod spec.nodeSelector to kubernetes.io/os: linux.
• Add a LabelKeyKubernetesOS constant for the well-known Kubernetes OS label.
• Update listener template merging logic to preserve the default nodeSelector unless the template explicitly sets one, with unit tests covering the merge scenarios.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
controllers/actions.github.com/resourcebuilder.go	Sets a default Linux NodeSelector on the listener pod and updates template merge behavior to avoid clearing the default when the template nodeSelector is nil.
controllers/actions.github.com/constants.go	Introduces LabelKeyKubernetesOS constant (kubernetes.io/os).
controllers/actions.github.com/resourcebuilder_test.go	Adds unit tests validating default nodeSelector behavior and template merge override/preserve scenarios.

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