Assistant: stop stale clients from resurrecting cleared threads

Root cause of the "chunks" clear bug: conversation history is
server-owned, but MAF seeds a thread from whatever the client sends
when no snapshot exists. Any stale tab (open across a clear or from an
older session) still holds the old conversation in memory, and its next
question re-persisted the whole thing — clears appeared to peel off one
layer at a time.

The agent middleware now drops client-sent history for threads with no
stored snapshot, keeping only the last user turn (fresh conversations
and empty-messages hydration requests pass through untouched). Verified
live: simulated a cleared-thread stale tab, asked from it, and only the
new turn was persisted.

Also: clearThread surfaces DELETE failures as a toast instead of
silently reloading, and conftest sets a dummy OPENAI_API_KEY so tests
can import app.main.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/tests/conftest.py

@@ -10,6 +10,9 @@ os.environ.setdefault(
 os.environ.setdefault("ADMIN_USERNAME", "testadmin")
 os.environ.setdefault("ADMIN_PASSWORD", "test-password")
 os.environ.setdefault("DATABASE_URL", "sqlite://")
+# app.main builds the MAF agent at import; the OpenAI client only needs a
+# syntactically-present key.
+os.environ.setdefault("OPENAI_API_KEY", "test-dummy-key")
 
 import pytest
 from sqlalchemy.pool import StaticPool