TPT-3501 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 client support and test coverage for the new read-only NodeBalancer field lke_cluster.

Changes:

• Added lke_cluster as a Property(json_object=...) on NodeBalancer.
• Introduced LKECluster as a JSONObject dataclass for decoding the nested object.
• Updated fixtures and unit tests to validate both populated and null lke_cluster responses.

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
linode_api4/objects/nodebalancer.py	Adds lke_cluster property mapping to decode the nested cluster reference.
linode_api4/common.py	Introduces LKECluster JSONObject schema used by NodeBalancer decoding.
test/fixtures/nodebalancers.json	Adds lke_cluster (object and null) to list fixture payload.
test/fixtures/nodebalancers_123456.json	Adds lke_cluster object to single NodeBalancer fixture payload.
test/unit/objects/nodebalancers_test.py	Extends list/get tests to assert lke_cluster decoding; fixes a docstring typo.

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