Add unit tests for rollout_utils

[GautamBytes]

What this PR does / why we need it

This PR adds a full unit test suite for rollout_utils.go to cover its critical helper functions. These functions, which handle pod filtering and partition calculations for CloneSet rollouts, previously had no test coverage, posing a risk for regressions.

The new tests use a mock Kubernetes client to ensure all logic can be verified reliably in an isolated environment. This improves the overall stability and code quality of the package.

How to test this PR

This PR consists of a new test file. The changes can be verified by running the test suite for the package and confirming that it passes:

go test ./pkg/internal/polymorphichelpers

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