[test-build] PluginManager shares plugin state across instances

[huanghuoguoguo]

Finding

PluginManager.plugins and PluginManager.plugin_handlers are defined as class-level mutable lists, while PluginManager.__init__ does not assign fresh instance lists. Creating multiple managers in the same Python process causes plugin state and handlers to leak between instances.

Why this matters

Tests or embedded/runtime scenarios that instantiate more than one PluginManager can observe stale plugins from a previous manager. This can produce incorrect plugin discovery, event emission, shutdown, or installation behavior.

Reproduction locked in tests

On the test-build branch:

uv run pytest tests/runtime/plugin/test_manager.py::test_plugin_manager_instances_should_not_share_plugin_state -q

The test is marked xfail(strict=True) to document the current bug without modifying SDK source in the test-build branch.

Expected behavior

Each PluginManager instance should own independent plugins and plugin_handlers lists, normally by initializing them in __init__.

[huanghuoguoguo]

已在 #65（commit 9723b59）修复。

修复方式：PluginManager.__init__ 现在为每个 manager 实例初始化独立的 self.plugins 和 self.plugin_handlers，不再共享类级列表状态。

回归用例：tests/runtime/plugin/test_manager.py::test_plugin_manager_instances_should_not_share_plugin_state。

测试：

• 对应 strict xfail 已取消，回归用例现在正常执行并通过。
• SDK：uv run pytest -> 269 passed, 10 warnings。
• LangBot master E2E：将本地 SDK editable 安装到 master worktree，WebUI 上传安装 qa/plugin-smoke，验证插件安装、Tool/Page 注册、Smoke Page iframe 渲染，以及 page-api /ping 返回 qa-plugin-smoke-page。