Send x-opencode-session header on API requests (required by OpenCode Go from 09/05)

Hi — we operate the OpenCode Go managed-inference API, and 1685 of your users' orgs call it through GitHub Copilot Chat (UA `GitHubCopilotChat/0.63.0`).

Starting 09/05, requests without an `x-opencode-session` header will error on our side — we need a stable per-conversation ID for routing/optimization. Copilot Chat doesn't send it in any version we've seen.

**Ask:** include `x-opencode-session: ` on all outbound inference HTTP requests. Any stable UUID per conversation works. Happy to confirm from our telemetry once a build ships it. Thanks!