Add Backend Configuration API

从 fastly_compute.requests.backend 和 stubs/wit_world/imports/backend.py 中的 WIT bindings 开始，了解现有的 backend 解析方式和可用操作。定义公开的 Backend wrapper、DynamicBackendConfig 以及 API design 中描述的 enum 映射，同时保留现有的 requests facade。使用 SDK 测试验证 wrapper 转换、introspection 和错误处理，并使用 Viceroy 或 mock endpoints 发送 backend 请求。

Overview

Extract and enhance backend functionality as a public API for both high-level and low-level use cases.

Current State:

• ✅ Basic backend resolution in fastly_compute.requests.backend (internal)
• ✅ Static backend lookup via wit_backend.Backend.open()
• ✅ Dynamic backend registration with TLS/timeout config
• ❌ Limited to requests facade - not exposed publicly
• ❌ Missing advanced features (client certs, custom CA, cipher selection, gRPC, pooling, keepalive)
• ❌ Returns raw WIT objects instead of wrapped resources

This Issue:
Create public fastly_compute.backend module exposing:

1. Backend wrapper - Pythonic wrapper around WIT Backend resource
2. Advanced configuration - All WIT features (certs, ciphers, TLS versions, keepalive, pooling)
3. Introspection API - Query backend properties (health, timeouts, TLS settings)
4. Requests integration - Existing facade continues working unchanged

WIT Interface

interface backend {
  use types.{error, open-error};
  use http-types.{tls-version};
  use secret-store.{secret};

  resource dynamic-backend-options {
    constructor();
    override-host: func(value: string);
    connect-timeout: func(value: u32);
    first-byte-timeout: func(value: u32);
    between-bytes-timeout: func(value: u32);
    use-tls: func(value: bool);
    tls-min-version: func(value: tls-version);
    tls-max-version: func(value: tls-version);
    cert-hostname: func(value: string);
    ca-certificate: func(value: string);
    tls-ciphers: func(value: string);
    sni-hostname: func(value: string);
    client-cert: func(client-cert: string, key: borrow<secret>);
    http-keepalive-time-ms: func(value: u32);
    tcp-keepalive-enable: func(value: u32);
    tcp-keepalive-interval-secs: func(value: u32);
    tcp-keepalive-probes: func(value: u32);
    tcp-keepalive-time-secs: func(value: u32);
    max-connections: func(value: u32);
    max-use: func(value: u32);
    max-lifetime-ms: func(value: u32);
    pooling: func(value: bool);
    grpc: func(value: bool);
    prefer-ipv6: func(value: bool);
  }

  type timeout-ms = u32;
  type timeout-secs = u32;
  type probe-count = u32;

  enum backend-health {
    unknown,
    healthy,
    unhealthy,
  }

  resource backend {
    open: static func(name: string) -> result<backend, open-error>;
    get-name: func() -> string;
    is-healthy: func() -> result<backend-health, error>;
    is-dynamic: func() -> result<bool, error>;
    get-host: func(max-len: u64) -> result<string, error>;
    get-override-host: func(max-len: u64) -> result<option<list<u8>>, error>;
    get-port: func() -> result<u16, error>;
    get-connect-timeout-ms: func() -> result<timeout-ms, error>;
    get-first-byte-timeout-ms: func() -> result<timeout-ms, error>;
    get-between-bytes-timeout-ms: func() -> result<timeout-ms, error>;
    is-tls: func() -> result<bool, error>;
    get-tls-min-version: func() -> result<option<tls-version>, error>;
    get-tls-max-version: func() -> result<option<tls-version>, error>;
    get-http-keepalive-time: func() -> result<timeout-ms, error>;
    get-tcp-keepalive-enable: func() -> result<bool, error>;
    get-tcp-keepalive-interval: func() -> result<timeout-secs, error>;
    get-tcp-keepalive-probes: func() -> result<probe-count, error>;
    get-tcp-keepalive-time: func() -> result<timeout-secs, error>;
  }

  register-dynamic-backend: func(prefix: string, target: string, options: dynamic-backend-options) -> result<backend, error>;
}

WIT bindings: stubs/wit_world/imports/backend.py

API Design

Extract backend logic from internal fastly_compute.requests.backend module into public fastly_compute.backend:

• Implement Backend resource wrapper around WIT Backend
• Static backends: Backend.open(name)
• Dynamic backends: Backend.create_dynamic(target, **options) for simple cases
• Advanced config: Backend.register_dynamic(name, DynamicBackendConfig(...)) with dataclass for all WIT options (TLS, certs, timeouts, pooling, keepalive, gRPC)
• Introspection properties: name, health, is_dynamic, host, port, connect_timeout (as timedelta), TLS settings
• Use TLSVersion enum (V1_0, V1_1, V1_2, V1_3) and BackendHealth enum (UNKNOWN, HEALTHY, UNHEALTHY)
• Existing requests facade continues working unchanged (uses new module internally)

Cross-SDK Comparison: Rust uses builder pattern for dynamic backends, Go uses simple factory functions with struct fields, JS uses constructor config object. Python should use DynamicBackendConfig dataclass (more Pythonic than builder).

Viceroy Testing

Viceroy supports backend configuration via viceroy.toml and dynamic backend registration. Tests can verify SDK wrapper logic (property conversion, enum mapping, error handling), but actual backend requests require mock server or real endpoints.

Reference

• Fastly Backends Documentation