[Fix] add streaming support in process_vllm_two_stage_request_discovered

[simondanielsson]

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Purpose

We might close this PR in favor of #114

Closes #113.

The service discovery path for vllm PD disaggregation does not support streaming, as opposed to the regular process_vllm_two_stage_request path (without service discovery) which does support streaming.

This PR adds support for response streaming also in the service discovery path. Without this, you cannot use the router with service discovery together with vllm bench serve --backend vllm which requires that streaming works.

Test Plan

Reproducer:

Test Result

Reproducer:

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[simondanielsson]

Closed in favor of this: #114