Add unit tests for parse_stripped_state_event

[guillemo12]

This PR adds a suite of tests for Synapse state events. It covers key scenarios around creation, updates and state consistency to prevent regressions in event processing and serialization. The primary goal is to increase test coverage and catch issues earlier in the development cycle.

Pull Request Checklist

• Pull request is based on the develop branch
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[reivilibre]

Thanks!

[reivilibre]

CI is failing, are you happy to look into it?

[guillemo12]

I think the issue was an improperly formatted line ending; I've just fixed it.