fix(sdk): always add queue label regardless of queue existence

[puwun]

What this PR does / why we need it:
create_job() skips adding the kueue.x-k8s.io/queue-name label when the specified local-queue doesn't exist, preventing Kueue from ever managing the job. Removed the conditional gate so the label is always added when queue_name is provided.

Changes:

• Removed the if queue_exists is not False condition that gated label addition, _check_queue_exists() is still called for its side effects (warning logs, error handling)
• Updated the "valid flow with non-existent queue" test case to expect the queue label

Which issue(s) this PR fixes:
Fixes #2775

[google-oss-prow]

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