fix(dashboard): keep memory.provider in the config schema so Desktop's dropdown survives (#63886)

The dashboard's dedicated memory-provider UI (4b184cbe5) excluded
memory.provider from /api/config/schema server-side. Desktop's settings
page builds its field list from that schema, so the Memory Provider
dropdown silently vanished from Desktop after v0.18.1.

- web_server.py: restore memory.provider as a select in _SCHEMA_OVERRIDES,
  with options built from plugins.memory discovery (was a stale hardcoded
  [builtin, honcho] list before the removal)
- plugins/memory: add list_memory_provider_names() — directory-scan-only
  name listing, safe at module import time (no provider imports)
- web ConfigPage: hide memory.provider client-side instead — the Plugins
  page owns the dedicated provider-switching UI there
- tests: schema contract (select present, category memory, builtin
  sentinel) + invariant that every discoverable provider is selectable

plugins/memory/__init__.py

@@ -143,6 +143,17 @@ def find_provider_dir(name: str) -> Optional[Path]:
 # Public API
 # ---------------------------------------------------------------------------
 
+def list_memory_provider_names() -> List[str]:
+    """Cheap name-only listing of discoverable memory providers.
+
+    Unlike :func:`discover_memory_providers`, this does NOT import provider
+    modules or run availability checks — it's a directory scan only, safe to
+    call at module-import time (e.g. when building the dashboard config
+    schema).
+    """
+    return sorted({name for name, _ in _iter_provider_dirs()})
+
+
 def discover_memory_providers() -> List[Tuple[str, str, bool]]:
     """Scan bundled and user-installed directories for available providers.