## Summary

When responses are streamed (`stream: true`), there is currently no standard way for a client to learn the proxy's authoritative cost for the completed request. The proxy already computes this cost server-side; it is just not delivered back to the client. As a result, clients tend to re-implement their own cost estimation from raw token counts, which cannot account for proxy-side modifiers (discounts, margins, MCP tool usage, prompt caching) that only the proxy knows about.

This issue proposes surfacing the proxy's already-computed cost to the client at stream end. Because the value is calculated synchronously before logging happens, this should be a relatively low-risk addition.

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## Background

A streaming request is delivered as a sequence of SSE events ending with `data: [DONE]`. Server-side, the proxy calculates the true cost accounting for:

- Model-specific token pricing (input, output, cached, reasoning) - Prompt caching discounts (Anthropic, OpenAI, Bedrock) - MCP tool usage costs - Operator-configured discounts (`cost_discount_config`) and margins (`cost_margin_config`) - Provider-specific uplifts (geo routing, speed tiers) - Built-in tool costs (web sear…