improve validation for R4 backport subscriptions in SubscriptionValidatingInterceptor

[fil512]

…eptorTest tests

also there are Msg.code FIXMEs

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.47%. Comparing base (497b9f2) to head (2f01b27). Report is 75 commits behind head on master.

:exclamation: Current head 2f01b27 differs from pull request most recent head cff1ed1

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5973 +/- ## ============================================ + Coverage 83.39% 83.47% +0.07% - Complexity 26927 27122 +195 ============================================ Files 1681 1692 +11 Lines 103965 104655 +690 Branches 13189 13254 +65 ============================================ + Hits 86702 87361 +659 - Misses 11613 11631 +18 - Partials 5650 5663 +13 ```

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