[Bug]: OpenCode API now requires `x-opencode-session` header

## QwenPaw Version

2.1.0

## Description

I have just received an email from OpenCode

> Some of your requests to OpenCode Go are missing an x-opencode-session header. If we don't have this we cannot properly optimize our service. Starting 09/06 requests missing this header may error.

> Here are your useragents that are missing this header:

> Python OpenAI client

It is believed the following PR is helpful

**Related PR(s):** #882

## Component(s) Affected

- [x] Core / Backend (app, agents, config, providers, utils, local_models)
- [ ] Console (frontend web UI)
- [ ] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Environment

- **QwenPaw version:** 2.1.0
- **OS:** Windows 11
- **Install method:** uv