feat(sdk): Refactoring using keyflow.init()

[Benjosh95]

Description

Problem:
The Key Flow unit tests were manually creating KeyFlow structs and setting fields directly instead of using the proper Init() function. This caused some further problems with the testing structure.

Solution:
Updated the tests to use the Init() function with proper KeyFlowConfig containing all required fields, and fixed timing issues in the continuous refresh tests by aligning token lifetimes, refresh intervals, and context durations to ensure predictable test behavior.

Jira-Ticket: https://jira.schwarz/browse/STACKITSDK-159

Checklist

• Issue was linked above
• No generated code was adjusted manually (check comments in file header)
• Changelogs
  • Changelog in the root directory was adjusted (see here)
  • Changelog(s) of the service(s) were adjusted (see e.g. here)
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[Benjosh95]

@Fyusel, could you please take a final look at the latest changes? If you approve them, I’ll go ahead and merge the pull request. :)