Skip stream_options injection for Anthropic Messages routes regardless of provider

[Copilot]

Streaming requests to /v1/messages routed through the Copilot provider (which fronts the Anthropic Messages API) get stream_options: { include_usage: true } injected into the body. The Messages endpoint rejects this field, causing upstream 400 stream_options: Extra inputs are not permitted errors.

Root cause

injectStreamOptions only skipped injection when provider was anthropic/gemini, or for the OpenAI /responses path. Requests with provider === 'copilot' and path /v1/messages matched neither guard, so injection proceeded into a body shape the Messages API rejects.

Fix

• containers/api-proxy/body-transform.js: Added a path-based guard (mirroring the existing /responses guard) that skips injection for /messages and /vN/messages routes regardless of provider, since the Messages API can be served by multiple providers (including Copilot).
• containers/api-proxy/body-transform.test.js: Added coverage confirming injection is skipped for Copilot on /v1/messages (and path variants), while still applied for Copilot on /v1/chat/completions.

// The Anthropic Messages API rejects stream_options.include_usage.
// Skip injection for /messages and /vN/messages routes regardless of
// provider, since the Copilot provider also fronts the Messages API.
if (/^\/?(?:v\d+\/)?messages(?:\/|$)/.test(pathOnly)) return null;

• Fixes api-proxy injects stream_options into Copilot Messages API (/v1/messages) requests, causing 400 #7201