chore(deps): httpx 全面迁移到 httpx2，SOCKS5 改用原生代理，升级 LLM SDK 大版本

- 依赖 httpx[socks] 换为 httpx2[socks]>=2.15，移除 httpx-socks / python-socks 及其 mypy 覆盖
- backend/src、ab_sdk、e2e 与单元测试统一使用 httpx2（异常类与 httpx 不通用）
- request_url 与 LLM 客户端工厂合并为 proxy_url()，SOCKS5 走 httpx2 原生代理（主机名由代理解析）
- 保留 #1104 过期连接池替换：httpcore2 2.15.1 的 start_tls 失败路径仍会泄漏隧道
- 日志同时压低 httpx2 与 httpx（google-genai、mcp 仍依赖旧 httpx）
- openai 3.35.1 / anthropic 1.15.0 / google-genai 2.31.0；结构化解析改用 chat.completions.parse
- ab_sdk 升到 0.6.0，客户端类型改为 httpx2.AsyncClient，同步插件文档

docs/en/dev/plugins/cli.md

@@ -3,8 +3,8 @@
 `ab-plugin` is installed with the `autobangumi-sdk` wheel. It depends only on `ab_sdk`. You do not need to install AutoBangumi.
 
 ```bash
-uv tool install ./autobangumi_sdk-0.5.0-py3-none-any.whl
-# or in a project: uv add ./autobangumi_sdk-0.5.0-py3-none-any.whl
+uv tool install ./autobangumi_sdk-0.6.0-py3-none-any.whl
+# or in a project: uv add ./autobangumi_sdk-0.6.0-py3-none-any.whl
 ```
 
 The wheel is an attachment of the 4.0 beta and stable GitHub releases. It is not on PyPI. Refer to [Get the SDK](/en/dev/plugins/sdk). The contract tests need pytest. The `pyproject.toml` that `new` creates already has pytest in its `dev` dependency group. For a project that you make yourself, use `uv add --dev pytest` (or install `autobangumi-sdk[test]`).
@@ -65,7 +65,7 @@ ab-plugin dev [path] [--config-dir config]
 ```bash
 ab-plugin new my-notify --kind notifier
 cd my-notify
-uv add ../autobangumi_sdk-0.5.0-py3-none-any.whl   # one time only
+uv add ../autobangumi_sdk-0.6.0-py3-none-any.whl   # one time only
 uv run pytest            # contract tests
 ab-plugin dev . --config-dir /path/to/autobangumi/config   # link and enable hot reload; restart AB one time
 # change code -> AB reloads -> check in the WebUI