refactor(agents): move MAC middleware impls out of shared kernel

knowledge_search, memory_injection and scoped_model_fallback no longer
belong in the cross-agent kernel (app/agents/shared/middleware): they are
consumed only inside multi_agent_chat. Relocate each impl next to the
builder that uses it:

- knowledge_search.py -> multi_agent_chat/shared/middleware/ (genuinely
  shared: its _render_priority_message feeds kb_context_projection, used by
  both the main agent and the KB subagent)
- memory_injection.py -> multi_agent_chat/shared/middleware/ (beside its
  memory.py builder)
- scoped_model_fallback.py -> multi_agent_chat/shared/middleware/resilience/
  (beside fallback.py/bundle.py)

Impls moved verbatim (git rename). Builders/consumers now import the local
sibling; main_agent knowledge_priority imports the new shared path; shared
middleware barrel trimmed.

Tests: repoint imports; convert the knowledge_search monkeypatch targets
from brittle dotted-string form to object-based patching (monkeypatch.setattr
on the imported module), which is robust to import ordering. No behavior
change.

surfsense_backend/tests/integration/retriever/test_knowledge_search_date_filters.py

@@ -8,7 +8,10 @@ from datetime import UTC, datetime, timedelta
 import numpy as np
 import pytest
 
-from app.agents.shared.middleware.knowledge_search import search_knowledge_base
+from app.agents.multi_agent_chat.shared.middleware import knowledge_search as ks
+from app.agents.multi_agent_chat.shared.middleware.knowledge_search import (
+    search_knowledge_base,
+)
 
 from .conftest import DUMMY_EMBEDDING
 
@@ -26,13 +29,9 @@ async def test_search_knowledge_base_applies_date_filters(
     async def fake_shielded_async_session():
         yield db_session
 
+    monkeypatch.setattr(ks, "shielded_async_session", fake_shielded_async_session)
     monkeypatch.setattr(
-        "app.agents.shared.middleware.knowledge_search.shielded_async_session",
-        fake_shielded_async_session,
-    )
-    monkeypatch.setattr(
-        "app.agents.shared.middleware.knowledge_search.embed_texts",
-        lambda texts: [np.array(DUMMY_EMBEDDING) for _ in texts],
+        ks, "embed_texts", lambda texts: [np.array(DUMMY_EMBEDDING) for _ in texts]
     )
 
     space_id = seed_date_filtered_docs["search_space"].id