Add a TestDiscoverAllPlugins class covering the six cases the recursive
scan needs to handle:
- flat plugin uses its manifest ``name:`` as the key
- category-namespaced plugin keys off ``<category>/<dirname>`` even when
the manifest ``name:`` is bare (regression test for the original bug —
``plugins/observability/langfuse/`` with ``name: langfuse`` must
surface as ``observability/langfuse``, not ``langfuse``)
- user-installed plugin overrides bundled on key collision
- depth cap: anything below ``<root>/<category>/<plugin>/`` is ignored
- bundled ``memory/`` and ``context_engine/`` are skipped (they have
their own loaders), but user plugins under those category names are
still scanned
Also add an in-source comment next to the key derivation pointing at the
loader's matching line (``PluginManager._parse_manifest`` in
plugins.py:1027-1028), so future renames of one site flag the other.
Both items raised in Copilot review on #27161.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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