test: build SDK quality test suite

[huanghuoguoguo]

Summary

Builds the SDK quality test suite and resolves the source issues that were recorded by that suite.

• Adds pytest/coverage configuration and CI test workflow with a 75% coverage gate.
• Adds unit and protocol-style tests across API entities, proxies, CLI commands, runtime IO, handlers, plugin containers, plugin manager behavior, discovery, and import utilities.
• Fixes the recorded SDK blockers found by the test-build suite.
• Removes the strict xfail markers for those blockers; the recorded issue tests now run normally and pass.

Fixed Issues

• Fixes [test-build] ComponentDiscoveryEngine shares component registry across instances #56: isolate ComponentDiscoveryEngine registries per instance.
• Fixes [test-build] importutil path rewriting breaks packages outside SDK source root #57: make import utilities derive module names from the target package/file location instead of rewriting arbitrary paths relative to the SDK source root.
• Fixes [test-build] Handler.call_action wraps peer errors twice #58: preserve peer ActionCallError messages instead of wrapping them twice.
• Fixes [test-build] Command error models have broken default/serialization contracts #59: make command error defaults and CommandReturn serialization safe.
• Fixes [test-build] ContentElement allows image_url type without image payload #60: validate ContentElement(type="image_url") requires an image_url payload.
• Fixes [test-build] PluginContainer.from_dict drops install metadata during roundtrip #61: preserve plugin install metadata during PluginContainer dict roundtrips.
• Fixes Runtime plugin handler cannot be imported directly due to circular import #62: break the runtime plugin handler circular import so it can be imported directly.
• Fixes [test-build] PluginManager shares plugin state across instances #63: isolate PluginManager plugin/plugin-handler state per instance.
• Fixes [test-build] PluginManager.emit_event reports emitted plugins twice #64: report each plugin only once from PluginManager.emit_event.

Validation

SDK Test Suite

uv run pytest
269 passed, 10 warnings

The previous blockers are no longer skipped or xfailed. rg "xfail|skip\\(" tests src returns no matches in this branch.

LangBot Master Smoke And E2E

Validated the SDK against a clean LangBot master worktree:

• LangBot master worktree: 767137a
• SDK under test: 9723b59
• Installed SDK with uv pip install -e /home/glwuy/langbot-app/langbot-plugin-sdk-test-build.
• Confirmed langbot_plugin.__file__ resolves to the local SDK source tree, not the locked/published package.

LangBot test commands:

uv run --no-sync pytest tests/unit_tests/plugin tests/unit_tests/provider/test_tool_manager.py tests/unit_tests/utils/test_pkgmgr.py tests/unit_tests/utils/test_importutil.py
129 passed, 9 warnings

uv run --no-sync pytest tests/integration/api/test_smoke.py tests/integration/api/test_pipelines.py
22 passed, 9 warnings

Browser e2e smoke:

• Started LangBot backend, plugin runtime, and WebUI from the master worktree with the local SDK installed.
• Built the local fixture plugin qa/plugin-smoke with the same SDK under test.
• Installed the plugin through WebUI Plugins -> Upload Local.
• Verified the plugin install task completed successfully.
• Verified plugin list/detail showed QA Plugin Smoke, Tool 1, and Page 1.
• Opened the plugin Smoke Page iframe and verified the sentinel text qa-plugin-smoke-page.
• Confirmed API diagnostics:
  • GET /api/v1/plugins includes initialized qa/plugin-smoke.
  • GET /api/v1/tools includes qa_echo.
  • POST /api/v1/plugins/qa/plugin-smoke/page-api for /ping returns qa-plugin-smoke-page.

Note: LangBot validation uses uv run --no-sync after installing the editable local SDK. Plain uv run may sync from uv.lock and replace the local SDK with the locked package, which would invalidate the test.