Fixed validation for VSR exact & regex subroutes

[jo-carter]

Since a VSR may only contain a single subroute when using exact or regex matching skipping route validation here is incorrect.

Correctly configured VSRs for regex or exact will be validated (and terminate) earlier in function, ie. where VS path is regex + only 1 subroute + subroute syntax check passes.

Resolves ticket 4727

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
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[haywoodsh]

Thank you @jo-carter for your PR! Could you please add some unit test cases for the validation failures?
https://github.com/nginxinc/kubernetes-ingress/blob/main/pkg/apis/configuration/validation/virtualserver_test.go#L821

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🔨 Latest commit	046ab97

[jo-carter]

@haywoodsh Sure, I've added a couple of unit tests.

[vepatel]

@jo-carter this change removes the capability of using regex or exact matches altogether

[jo-carter]

@haywoodsh Do we need to squash / amend any of these commits ? They undo each other in places (the unit tests).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8a2c9a9) 52.32% compared to head (4e57469) 52.35%.
Report is 6 commits behind head on main.

❗ Current head 4e57469 differs from pull request most recent head 046ab97. Consider uploading reports for the commit 046ab97 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4744      +/-   ##
==========================================
+ Coverage   52.32%   52.35%   +0.02%     
==========================================
  Files          61       61              
  Lines       17502    17505       +3     
==========================================
+ Hits         9158     9164       +6     
+ Misses       8015     8012       -3     
  Partials      329      329              

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[ADubhlaoich]

LGTM for docs changes: not approving as most of the PR content is code-based.