[test-build] ComponentDiscoveryEngine shares component registry across instances

[huanghuoguoguo]

Background

While building the SDK test-build coverage branch, a contract test showed that ComponentDiscoveryEngine.components is shared across engine instances.

Reproducer

first = ComponentDiscoveryEngine()
second = ComponentDiscoveryEngine()
first.load_component_manifest('tool.yaml', no_save=False)
assert second.get_components_by_kind('Tool') == []

Expected: each discovery engine instance should have an isolated registry.

Actual: second can see components loaded by first, because components is defined as a class variable.

Risk

• Multiple plugin/blueprint discovery runs in one process can leak component state into each other.
• Tests become order-dependent.
• Plugin reload or multi-plugin scans may produce duplicate or stale component lists.

Test Signal

Locked in test-build as an xfail(strict=True) contract test:

tests/utils/test_discovery.py::test_component_registry_should_be_isolated_per_engine_instance

No source fix is included in the test-build branch.

[huanghuoguoguo]

已在 #65（commit 9723b59）修复。

修复方式：ComponentDiscoveryEngine.__init__ 现在为每个实例初始化独立的 self.components = {}，避免类属性里的可变 dict 在多个 discovery engine 实例之间共享。

回归用例：tests/utils/test_discovery.py::test_component_registry_should_be_isolated_per_engine_instance。

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。