Replace connexion with fast-api: initialization, constants, exceptions, schemas.

[potiuk]

See parent description

Depends on #60944, #60946

[Prab-27]

@potiuk I'd like to work on this issue

[Varshitha9115]

Hi @potiuk 👋
I’d like to work on this issue if it’s still available.
I’ve reviewed the parent issue and related tasks and am happy to follow the recommended approach.
Please let me know if there are any specific guidelines before I start.
Thanks!