feat: invoke authorized OpenRouter text streams

[sjungwon03]

Problem

OpenGranter can validate an already fetched OpenRouter SSE response, but no internal invoker sends an authorized stream:true request to OpenRouter. A streaming invoker must use the same fixed endpoint, secret, model/provider scope, request snapshots and timeout safety as the current non-streaming adapter, then pass its response through the bounded SSE validator.

Scope

Add an internal text-only delegated OpenRouter streaming invoker. Share request validation/body construction with the non-streaming invoker so supported text and sampling controls preserve their existing semantics. Capture the approved model/provider attempt before any await; reject function-tool controls that the text-only stream decoder cannot represent. Send one POST to the fixed OpenRouter chat URL with stream:true, exact provider.only, a server-held Bearer key, redirect rejection and bounded timeout. Await validated delta delivery through the existing response boundary. Return final normalized usage only on complete terminal/usage/done; classify pre-header and midstream failures safely without retry or leaking raw errors.

Do not expose client stream:true, write usage/audit, add direct-provider streaming, or change route orchestration. The future gateway integration must apply authentication, IAM, limits and audit before calling this invoker.

Acceptance

• Authorized text request reaches only the fixed endpoint with a captured model/provider set and existing supported controls; callback receives validated deltas in order and completion returns final usage.
• Invalid attempts, unsupported tool controls, malformed requests and missing credentials stop before HTTP. Scope changes during credential resolution cannot alter the request or accepted response.
• HTTP status, malformed/incomplete SSE, timeout, redirect/network and callback failures retain fixed safe categories and possible-billing metadata; no upstream body/key enters errors.
• Existing non-streaming adapter behavior and full checks remain green. Refs feat: complete OpenRouter external-client compatibility #116.