[Task]: Enable the integration tests for openrouter

[rdimitrov]

Description

This is just about adding the openrouter provider to the list of providers and adding the API token as a secret.

Additional Context

No response

[jhrozek]

ah, thank you, I meant to ping you tomorrow when I'm back from the 1-day break. Feel free to ping me if anything doesn't work, but at least locally it was enough to run the integration tests with the key exported as an evironment variable. FWIW, I don't recommend running the reasoning models for tests if we need to run them fast, on the other hand, it would be nice to have some test coverage for a reasoning model especially if we also fix #977