fix(instancemanager): honor node selector setting while setup the pod

[COLDTURNIP]

Which issue(s) this PR fixes:

Issue longhorn/longhorn#12834

What this PR does / why we need it:

Special notes for your reviewer:

Additional documentation or context

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[Copilot]

Pull request overview

This PR addresses longhorn/longhorn#12834 by ensuring InstanceManager pod creation honors the system-managed-components-node-selector setting, avoiding rapid failure/recreate loops when the selected node doesn’t satisfy the selector.

Changes:

• Add a new InstanceManager condition reason for node selector mismatch (NodeSelectorNotMatching).
• Check whether the target node matches the system-managed node selector before deciding pod deletion/recreation and before creating a pod.
• Introduce a helper (isNodeSelectorMatchingForIM) to evaluate selector vs. Kubernetes Node labels.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
k8s/pkg/apis/longhorn/v1beta2/instancemanager.go	Adds a new condition reason constant used to report node selector mismatch.
controller/instance_manager_controller.go	Adds selector-vs-node matching logic to prevent creating IM pods on nodes that don’t satisfy the configured selector.

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[derekbit]

@COLDTURNIP Please address the comments from Copilot. Thanks.