fix(console): stream zen request bodies

[thdxr]

Summary

• stream Zen request bodies upstream after bounded prefix inspection
• patch provider model without materializing the complete JSON body
• remove provider retries and format conversion for the single-provider path
• infer response streaming from upstream content type

Verification

• bun run typecheck in packages/console/app
• bun test test/requestBody.test.ts test/rateLimiter.test.ts
• repository pre-push typecheck

Rollout

Cloudflare previously regressed with synthetic request streams, so monitor exceededMemory immediately after production deployment.