fix: default HPA metrics for fixed replica bounds

[labuladong]

A Function with minReplicas=maxReplicas=1 and no explicit autoscaling policy gets an HPA with no metrics. Kubernetes defaults it to 80% CPU utilization, while the operator continues comparing it with an empty desired metrics list. The workload and HPA can be healthy, but the Function remains HPAReady=False with Action=Update.

Use the existing default metrics selection when the replica bounds are equal as well. Replica bounds remain fixed. This one-condition change applies through the shared HPA generator used by Functions, Sources, and Sinks.

Validation:

• Observed the mismatch on a fresh AWS v2 test CE: the Function successfully transformed a Pulsar message, its Pod was 2/2 Ready, and the HPA reported ScalingActive=True, but the Function retained HPAReady=False/Update.
• Added a regression test that models API-server defaulting; it fails before the fix and passes after it.
• go test ./controllers/spec -count=1
• Reviewed the diff for scope and simplification; git diff --check passes.

No controller deployment or shared control-plane change was made. Runtime validation of a rebuilt operator remains pending.

[github-actions]

@labuladong:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)