Fix server handler Sendability warnings

[czechboy0]

Motivation

Fixes a warning emitted for each operation method in generated server code, which looked like:

Converting non-sendable function value to '@Sendable (APIHandler) -> ((Operations.listPets.Input) async throws -> Operations.listPets.Output)' may introduce data races

Modifications

Instead of passing the function itself, we now invoke it in a closure.

Result

The warning has gone away.

Test Plan

Adapted integration tests.