viable/strict/1792049632: [TokenSwitch] Specify the backend contract and test it in CI (#198963)

发生了什么

We're adding a ROCm MoRI backend for TokenSwitch. While working on the implementation, it baked some implicit semantics from the NCCL EP behavior into TokenSwitchNCCL. This PR establishes the API and behavior contract on its own, with new CI coverage, that will pave the way for the new TokenSwitchMoRI implementation. Claude (AI assistant) drafted the following description of the change: Why. TokenSwitch's semantics…

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