fix(opencode): remove Qwen sampling defaults

[opencode-agent]

Summary

• stop forcing temperature: 0.55 and top_p: 1 for every Qwen model
• leave Qwen sampling unset so provider or server defaults apply, while chat.params plugins can still override all sampling controls
• add regression coverage for Qwen3.8 and Qwen3-Coder model IDs

Why

Qwen sampling recommendations now vary by model family and thinking mode, so the blanket name-based defaults are stale and can override better provider-side settings, especially for custom and local endpoints.

Fixes #42775

Testing

• bun test test/provider/transform.test.ts (411 pass)
• Prettier check on changed files
• git diff --check
• bun typecheck attempted, but tsgo was terminated by shared-host resource exhaustion

Requested by: @rekram1-node (Aiden via Slack)