Add unit tests for util functions

[GautamBytes]

What this PR does / Why we need it
The utility functions in pkg/webhook/util/util.go are responsible for configuring the webhook from environment variables. These functions previously lacked unit test coverage, making it hard to verify their behavior or prevent future regressions.

This PR adds unit tests to ensure this critical configuration logic is reliable and works as expected.

Key Scenarios Covered

• Functions correctly return a default value when an environment variable is not set.
• Functions correctly return the value from a set environment variable.
• The GetPort function is tested for both its default behavior and for correctly parsing a valid integer from the WEBHOOK_PORT environment variable.

[kruise-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.42%. Comparing base (00aae12) to head (3bf25d3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
- Coverage   47.52%   47.42%   -0.11%     
==========================================
  Files          63       65       +2     
  Lines        9799     9890      +91     
==========================================
+ Hits         4657     4690      +33     
- Misses       4585     4643      +58     
  Partials      557      557              

Flag	Coverage Δ
unittests	47.42% <ø> (-0.11%)	⬇️

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[GautamBytes]

@furykerry can you help me with why 'codecov/project' fails saying low coverage. it seems to be 100% as i can check on terminal

gautam@Gautam:/mnt/e/rollouts$ go tool cover -func=util.cover | grep "util.go"
github.com/openkruise/rollouts/pkg/webhook/util/util.go:26:     GetHost         100.0%
github.com/openkruise/rollouts/pkg/webhook/util/util.go:30:     GetNamespace    100.0%
github.com/openkruise/rollouts/pkg/webhook/util/util.go:37:     GetSecretName   100.0%
github.com/openkruise/rollouts/pkg/webhook/util/util.go:44:     GetServiceName  100.0%
github.com/openkruise/rollouts/pkg/webhook/util/util.go:51:     GetPort         100.0%
github.com/openkruise/rollouts/pkg/webhook/util/util.go:63:     GetCertDir      100.0%
github.com/openkruise/rollouts/pkg/webhook/util/util.go:70:     GetCertWriter   100.0%