We recently introduced a new testing framework in #136799 that enables testing HPA behavior using an External Metrics mock server.

Currently, the framework includes only a single test case (basic scale up and scale down), which is not sufficient to fully validate HPA behavior. We should expand the test coverage to ensure the framework works reliably across more realistic scenarios.

We propose adding the following test cases:

• Multiple Metrics: Verify that a single HPA can correctly handle two or more external metrics simultaneously.
• Stabilization Window: Ensure that the HPA does not scale too aggressively when metric values fluctuate rapidly, respecting the configured stabilization window.
• Scaling Limits: Validate that the configured scaling limits (behavior.scaleUp and behavior.scaleDown) are enforced correctly.

/sig autoscaling
/good-first-issue
/kind cleanup