Revoke JWT token on provider-level /auth/logout routes

[securylight]

Core's /auth/logout (#67289) revokes the current JWT before redirecting to the auth manager's logout URL. FabAuthManager's and KeycloakAuthManager's own /auth/logout routes don't do the same before redirecting to the IdP, so a token stays valid if that route is reached directly. This adds the same revoke_token() call to both, for consistency.

Test plan

• Added test_logout_revokes_token / test_logout_without_token_cookie_does_not_revoke for both providers
• Existing logout tests still pass

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Sonnet 5)

Generated-by: Claude Code (Sonnet 5) following the guidelines

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[vincbeck]

This is done even if the auth manager has its custom logout route. Why doing it twice? https://github.com/apache/airflow/blob/main/airflow-core/src/airflow/api_fastapi/core_api/routes/public/auth.py#L65