TPT-3499: Support NodeBalancer lke_cluster field

[jbilskiAkam]

📝 Description

This PR adds support for the new read-only field lke_cluster in the NodeBalancer struct.

✔️ How to Test

To run relevant unit tests use:

make test-unit

[Copilot]

Pull request overview

Adds support in linodego for the new read-only lke_cluster field returned on NodeBalancer objects, along with unit-test coverage and updated fixtures.

Changes:

• Add LKECluster field to NodeBalancer and introduce NodeBalancerLKECluster response struct.
• Extend unit tests to validate unmarshalling for both null and populated lke_cluster.
• Update/add JSON fixtures to include the new lke_cluster field.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
nodebalancer.go	Adds NodeBalancer.LKECluster plus the NodeBalancerLKECluster type for response unmarshalling.
test/unit/nodebalancer_test.go	Expands GET/LIST tests to assert LKECluster is correctly decoded.
test/unit/fixtures/nodebalancers_list.json	Updates list fixture to include lke_cluster for both null and object cases.
test/unit/fixtures/nodebalancer_get.json	Updates get fixture to include "lke_cluster": null.
test/unit/fixtures/nodebalancer_get_with_lke_cluster.json	New fixture representing a NodeBalancer with a populated lke_cluster.

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