[Feat]: finished by finish reason

[scarlet25151]

Pull Request Description

[Please provide a clear and concise description of your changes here]

Related Issues

Resolves: #2013

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[gemini-code-assist]

Code Review

This pull request introduces a short-circuiting mechanism for prefill requests within the PD disaggregation algorithm. It enables the gateway to return an immediate response to the client if a specific finish reason is detected during the prefill phase, effectively bypassing subsequent processing steps. Key changes include modifications to the routing context, HTTP execution flow, and metrics handling, as well as the addition of utility functions for constructing Envoy-compatible immediate responses and new unit tests. I have no feedback to provide.