fix(backend/copilot): prevent title update from overwriting session messages (#12302)

### Changes 🏗️

Fixes a race condition in `update_session_title()` where the background
title generation task could overwrite the Redis session cache with a
stale snapshot, causing the copilot to "forget" its previous turns.

**Root cause:** `update_session_title()` performs a read-modify-write on
the Redis cache (read full session → set title → write back). Meanwhile,
`upsert_chat_session()` writes a newer version with more messages during
streaming. If the title task reads early (e.g., 34 messages) and writes
late (after streaming persisted 101 messages), the stale 34-message
version overwrites the 101-message version. When the next message lands
on a different pod, it loads the stale session from Redis.

**Fix:** Replace the read-modify-write with a simple cache invalidation
(`invalidate_session_cache`). The title is already updated in the DB;
the next access just reloads from DB with the correct title and
messages. No locks, no deserialization of the full session blob, no risk
of stale overwrites.

**Evidence from prod logs (session `41a3814c`):**
- Pod `tm2jb` persisted session with 101 messages
- Pod `phflm` loaded session from Redis cache with only 35 messages (66
messages lost)
- The title background task ran between these events, overwriting the
cache

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] `poetry run pytest backend/copilot/model_test.py` — 15/15 pass
  - [x] All pre-commit hooks pass (ruff, black, isort, pyright)
- [ ] After deploy: verify long sessions no longer lose context on
multi-pod setups

autogpt_platform/backend/backend/copilot/model.py

@@ -705,19 +705,10 @@ async def update_session_title(session_id: str, title: str) -> bool:
             logger.warning(f"Session {session_id} not found for title update")
             return False
 
-        # Update title in cache if it exists (instead of invalidating).
-        # This prevents race conditions where cache invalidation causes
-        # the frontend to see stale DB data while streaming is still in progress.
-        try:
-            cached = await _get_session_from_cache(session_id)
-            if cached:
-                cached.title = title
-                await cache_chat_session(cached)
-        except Exception as e:
-            # Not critical - title will be correct on next full cache refresh
-            logger.warning(
-                f"Failed to update title in cache for session {session_id}: {e}"
-            )
+        # Invalidate the cache so the next access reloads from DB with the
+        # updated title. This avoids a read-modify-write on the full session
+        # blob, which could overwrite concurrent message updates.
+        await invalidate_session_cache(session_id)
 
         return True
     except Exception as e: