feat: reduce create Provider boilerplate by using declarative macro

- [x] I have looked for existing issues (including closed) about this

## Feature Request

Currently all provider integrations manually implement exact same boilerplate (`Ext`, `Builder`, `Capabilities`, etc.). This results in hundreds of lines of code (~15 providers * ~50 lines each) and will keep increasing as new models are added.

### Motivation

Significant reduction in code duplication, easier to add new models

### Proposal

Create a `define_provider!` or `setup_provider` declarative macro to generate this boilerplate in a single line, example:

```rust
define_provider!(MoonshotExt, MoonshotBuilder, "MOONSHOT_API_KEY", "https://api.moonshot.cn/v1", "/models");
```

### Alternatives

Look at existing provider implementations (e.g., MoonshotExt, MoonshotBuilder) to understand the boilerplate pattern. The macro should generate Ext, Builder, Capabilities, etc. Start by exploring the provider module structure and Rust's declarative macro system. Test the macro by applying it to a new or existing provider to ensure it generates correct code.

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