The existing hook tests call the plugin's _on_* callbacks directly, which passes
even if the bundled plugin stops auto-loading or a hook name drifts from what core
fires — real runs would go dark while the suite stays green.
Add test_plugin_e2e.py, which drives the real dispatch chain through public entry
points only (discover_plugins -> invoke_hook -> registered callback -> emitter ->
tel_* tables), exactly as core does:
- one completed turn produces tel_runs / tel_model_calls / tel_tool_calls rows
with real provider/model/tool values and correct counts;
- telemetry.local=false means the plugin does not load and nothing is written.
Verified robust against test ordering (singleton resets for the plugin manager and
the emitter in the fixture).
