fix(memory): don't let an unreadable store get overwritten with an empty one (#5831)

* fix(memory): don't let an unreadable store get overwritten with an empty one

load_all() answered a failed read the same way it answered an empty store:
with []. Every mutation path is a read-modify-write (load the whole file,
change it, save it back), so a failed read became

    load_all() -> []  ->  [].append(new)  ->  save([new])

and save() is atomic, so the replacement stuck.

The case that actually destroys data is a store that is READABLE but not
parseable - a truncated file, or one holding {} instead of []. Nothing
obstructs the write, so adding a memory returns HTTP 200 and every memory
already stored is gone. Verified end-to-end against a running instance: on the
current code a truncated memory.json plus one add leaves the file holding only
the new entry. Truncation is reachable - core/database.py rewrites memory.json
during migration with a plain open(.., "w") + json.dump, which is not atomic.

A live exclusive lock is not the dangerous case: it blocks the read and the
os.replace alike, so the save fails too and the store survives. That path
currently 500s and loses nothing.

_read_entries() now returns [] only when the file genuinely does not exist and
raises MemoryStoreUnreadable for every other failure, including a store that
parses but is not a JSON array. load_all() keeps the old lenient behaviour so
display, search and context injection still degrade quietly instead of
breaking chat. The read-modify-write callers switch to load_all_for_update(),
which propagates the error: the memory routes turn it into a 503 and change
nothing, backup import refuses rather than saving only the incoming rows, and
auto-extraction and the audit merge skip the write. The audit merge mattered
most - it rebuilds the whole file from one owner's slice plus everyone else's
rows, so an empty read there dropped every other tenant's memories.

The corrupt-JSON path still gets its one shot at the legacy memory.txt
migration before raising, so that recovery is unchanged.

The two updated fakes gained load_all_for_update because the real class has it;
MagicMock would otherwise hand the import path a Mock instead of the seeded list.

Fixes #5673

* fix(memory): fail closed on the remaining read-modify-write add paths

The strict loader landed with the routes, the backup import and the extractor
converted, but three read-modify-write sinks still called load_all(), which
degrades an unreadable store to []. Two of them are the paths users actually
reach, so the data loss in #5673 stayed reproducible:

- src/ai_interaction.py do_manage_memory, action "add" — reached from ordinary
  chat via src/tool_execution.py:793 -> dispatch_ai_tool. "Remember that I
  prefer X" against an unreadable store wrote a one-entry file over it and
  reported success.
- mcp_servers/memory_server.py, action "add" — the same shape through
  _scope_entries(), registered as a built-in in src/builtin_mcp.py.
- src/memory_provider.py NativeMemoryProvider.remember and .delete — wired
  into app state in src/app_initializer.py but not consumed outside tests yet,
  converted here so the pattern is uniform before it goes live.

The MCP server takes _scope_entries(for_update=True) so list keeps the lenient
read. The edit and delete branches on both tool paths were already fail-closed
by accident — an empty view matches nothing and returns before the save — so
they are left alone.

The three new tests drive the real entry points rather than replaying the
shape, and use a truncated store, which is the case that reads back fine so
nothing stops the save. Each asserts memory.json is byte-identical afterwards;
all three fail on the previous commit with the store overwritten.

mcp_servers/memory_server.py

@@ -17,6 +17,8 @@ from mcp.types import Tool, TextContent
 
 sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
+from src.memory import MemoryStoreUnreadable
+
 server = Server("memory")
 
 # Late-initialized managers (set during first tool call)
@@ -29,6 +31,10 @@ _OWNER_SCOPE_ERROR = (
     "Error: Memory MCP owner is not configured for an owner-scoped memory store. "
     "Set ODYSSEUS_MCP_MEMORY_OWNER for this server or use the owner-aware native memory tool."
 )
+_UNREADABLE_STORE_ERROR = (
+    "Error: Memory store is temporarily unreadable — nothing was saved. "
+    "Repair or restore memory.json, then retry."
+)
 
 
 def _configured_owner() -> str | None:
@@ -51,9 +57,21 @@ def _owner_scoped_store(entries: list[dict]) -> bool:
     return any(_entry_owner(entry) for entry in entries if isinstance(entry, dict))
 
 
-def _scope_entries() -> tuple[str | None, list[dict], list[dict], str | None]:
-    """Return configured owner, all entries, visible entries, and optional error."""
-    entries = _memory_manager.load_all()
+def _scope_entries(for_update: bool = False) -> tuple[str | None, list[dict], list[dict], str | None]:
+    """Return configured owner, all entries, visible entries, and optional error.
+
+    ``for_update=True`` is for read-modify-write callers. They save the ``all
+    entries`` list back, so an unreadable store must be reported as an error
+    instead of degrading to ``[]`` — otherwise the save writes their one new
+    entry over the whole store (issue #5673).
+    """
+    if for_update:
+        try:
+            entries = _memory_manager.load_all_for_update()
+        except MemoryStoreUnreadable as e:
+            return None, [], [], f"{_UNREADABLE_STORE_ERROR} ({e})"
+    else:
+        entries = _memory_manager.load_all()
     owner = _configured_owner()
     if owner is None and _owner_scoped_store(entries):
         return None, entries, [], _OWNER_SCOPE_ERROR
@@ -161,7 +179,7 @@ async def call_tool(name: str, arguments: dict) -> list[TextContent]:
         category = arguments.get("category", "fact")
         if not text:
             return _text_result("Error: Memory text cannot be empty")
-        owner, memories, _visible, scope_error = _scope_entries()
+        owner, memories, _visible, scope_error = _scope_entries(for_update=True)
         if scope_error:
             return _text_result(scope_error)
         entry = _memory_manager.add_entry(text, source="ai_agent", category=category, owner=owner)