[Router] Response augmentation + block handling (Mode B)

[ramkrishna2910]

Goal

Response augmentation + block handling for Mode B.

Scope

• Add x_lemonade_route object to the chat.completion response + an x-lemonade-route header.
• verdict=block → synthesized refusal, finish_reason:"content_filter", no backend call.
• Streaming variant: chat.completion.chunk + header.

Acceptance

• Non-stream, stream, and block paths tested.
• A vanilla OpenAI client ignoring the extra field still works unchanged.

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Depends on: #2385, #2376