fix(mcp): guard tool_status/list_wings/list_rooms/get_taxonomy against None metadata

Four more MCP handlers iterate a metadata list and call m.get(...)
unconditionally. When the cache contains a None entry (drawers with no
metadata, common on older mining paths), the try block catches the
AttributeError and marks the response "partial: true" with an
error message — visible as {"error": "'NoneType' object has no
attribute 'get'", "partial": true} returned from mempalace_status even
though the palace data is otherwise fetchable.

Same m = m or {} guard we applied to searcher.py (d3a2d22, a51c3c2)
and miner.status() (66f08a1). None-metadata drawers now roll up under
the existing "unknown" fallback bucket instead of poisoning the
response with a misleading partial flag.

Regression test: mock the metadata cache with a None in the middle,
assert tool_status returns clean counts and no error/partial fields.
Verified the test fails without the guard.

998 tests pass.

mempalace/mcp_server.py

@@ -315,6 +315,7 @@ def tool_status():
     try:
         all_meta = _get_cached_metadata(col)
         for m in all_meta:
+            m = m or {}
             w = m.get("wing", "unknown")
             r = m.get("room", "unknown")
             wings[w] = wings.get(w, 0) + 1
@@ -368,6 +369,7 @@ def tool_list_wings():
     try:
         all_meta = _get_cached_metadata(col)
         for m in all_meta:
+            m = m or {}
             w = m.get("wing", "unknown")
             wings[w] = wings.get(w, 0) + 1
     except Exception as e:
@@ -391,6 +393,7 @@ def tool_list_rooms(wing: str = None):
         where = {"wing": wing} if wing else None
         all_meta = _fetch_all_metadata(col, where=where)
         for m in all_meta:
+            m = m or {}
             r = m.get("room", "unknown")
             rooms[r] = rooms.get(r, 0) + 1
     except Exception as e:
@@ -409,6 +412,7 @@ def tool_get_taxonomy():
     try:
         all_meta = _get_cached_metadata(col)
         for m in all_meta:
+            m = m or {}
             w = m.get("wing", "unknown")
             r = m.get("room", "unknown")
             if w not in taxonomy: