[Core] Consolidate Operation ID generation

[richardwhiuk]

Minor core code refactor spotted while doing other development.

Currently we split Operation ID generation across two functions: getOrGenerateOperationId and fromOperation

This refactor moves all of the code into getOrGenerateOperationId

This has a knock-on effect, which means that callback operation IDs now get correctly created with a camel case name - I think this is an improvement.

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Core Team for review: @wing328 , @jimschubert , @cbornet , @jmini , @etherealjoy

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/10350469557/job/28647111728?pr=19339

looks like the refactoring results in some changes in the samples, can you please take a look when you've time?

[wing328]

cc @OpenAPITools/generator-core-team

[wing328]

the refactoring looks good. thanks for the PR