Fixing multiple problems with SpringCodegen when activating ServerSentEvents

[kgeilmann]

This PR fixes a number of NPEs when using the spring generator with the options 'reactive' and 'serverSentEvents'.

It fixes #17271 and some similar problems, see commit 31a705c for all identified and fixed problems. The problems mainly originated from not handling $ref correctly and expecting to always have the text/event-stream media type (which is obviously not true for non-sse operations).

Additionally a sample was added, to ensure the 'petstore-with-fake-endpoints-models-for-testing' (extended with a simple sse operation) can be generated without problems.

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

can you please add the new output folder to the github workflow so that it will be tested moving forward?

https://github.com/OpenAPITools/openapi-generator/blob/master/.github/workflows/samples-spring.yaml
https://github.com/OpenAPITools/openapi-generator/blob/master/.github/workflows/samples-spring-jdk17.yaml

[kgeilmann]

Added.

Fixed the email as well, thanks for pointing it out

[wing328]

cc @cachescrubber (2022/02) @welshm (2022/02) @MelleD (2022/02) @atextor (2022/02) @manedev79 (2022/02) @javisst (2022/02) @borsch (2022/02) @banlevente (2022/02) @Zomzog (2022/09) @martin-mfg (2023/08)

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/13336520230/job/37276957271?pr=20658

please update the samples one more time when you've time

UPDATE: done via 5b3014a

[wing328]

cc
@cachescrubber (2022/02) @welshm (2022/02) @MelleD (2022/02) @atextor (2022/02) @manedev79 (2022/02) @javisst (2022/02) @borsch (2022/02) @banlevente (2022/02) @Zomzog (2022/09) @martin-mfg (2023/08)