feat: inject session_id for OpenRouter session grouping

Problem

OpenRouter supports session-level grouping via a session_id field in the request body. This allows upstream providers to attribute and group requests by session, which is useful for analytics, billing, and debugging across multi-turn conversations.

Currently, opencode does not pass session_id to OpenRouter requests, so all requests from the same conversation are ungrouped at the provider level.

Proposed solution

Inject session_id into the request extraBody when the model API provider is @openrouter/ai-sdk-provider. This leverages the existing extraBody mechanism in the AI SDK to pass the field through without requiring provider-specific request restructuring.

Scope

• packages/opencode/src/provider/transform.ts — add extraBody.session_id for OpenRouter models
• packages/opencode/test/provider/transform.test.ts — test coverage