Add first-class Qwen preset

[yuanhao]

Closes #38.

Summary

• Add first-class hosted Qwen / DashScope support via ModelConfig::qwen(...).
• Add OpenAiCompat::qwen() for Qwen reasoning-content parsing and OpenAI-compatible max-token behavior.
• Add ModelConfig::openai_compat(...) for local/custom deployments that need explicit model-family compat flags.
• Document hosted Qwen, local Qwen, regional DashScope URLs, and combined Qwen-on-Ollama compat.

Impact

• Additive public API only.
• Existing provider behavior is unchanged.
• CLI now supports --provider qwen, and DASHSCOPE_API_KEY is only preferred for Qwen.

Validation

• cargo fmt --check
• cargo check --examples
• cargo test provider::model
• cargo test provider::openai_compat
• cargo test