[test-build] PluginManager.emit_event reports emitted plugins twice

[huanghuoguoguo]

Finding

PluginManager.emit_event appends a plugin to emitted_plugins inside the if resp["emitted"] block and then appends the same plugin again unconditionally. When a plugin reports emitted=True, callers receive duplicate plugin entries.

Why this matters

Consumers of emit_event may use the returned plugin list for observability, ordering, audit records, or follow-up logic. Duplicates make the returned result inaccurate even though the event was only emitted once.

Reproduction locked in tests

On the test-build branch:

uv run pytest tests/runtime/plugin/test_manager.py::test_emit_event_should_report_each_emitting_plugin_once -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 emitting plugin should appear once in emitted_plugins. Non-emitting plugins should not be appended.

[huanghuoguoguo]

已在 #65（commit 9723b59）修复。

修复方式：PluginManager.emit_event 删除了无条件追加 plugin 的重复 append，只在 handler 实际 emitted 时记录一次。

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

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。