fix(chat): track chat hot-path background tasks for strong references (#4443) (#4444)

Two background tasks scheduled on every chat completion in
routes/chat_helpers.py — the memory/skill extraction dispatch and the
session auto-namer — are created via bare asyncio.create_task(...).
asyncio only holds a weak reference to the outer task, so the GC can
collect it mid-execution and the work silently never runs.

Add a module-private _BG_TASKS set and a _spawn_bg() helper that mirrors
WebhookManager._spawn_tracked (the pattern #3964 / #4336 established for
the webhook emitters two lines apart in the same function). Route both
call sites through it so the lifecycle owner is explicit.

Adds an AST-level guard test that fails on any bare
asyncio.create_task(...) statement in routes/chat_helpers.py to prevent
a regression — same shape as test_webhook_emitters_use_manager.py from
#4336.

The same bare pattern exists in routes/email_routes.py and
routes/cookbook_routes.py; left out of this PR per CONTRIBUTING.md's
"one fix per PR" and tracked in #4443's "Additional Information" for a
follow-up.

tests/test_chat_helpers_bg_tasks_tracked.py

@@ -0,0 +1,71 @@
+"""Guard: chat hot-path background tasks must go through _spawn_bg.
+
+asyncio only holds a weak reference to a bare create_task() result, so the
+GC can collect the outer task before its body runs and the background work
+(memory/skill extraction, session auto-naming) silently never happens.
+routes/chat_helpers.py owns these schedules via _spawn_bg(), which adds the
+task to _BG_TASKS and discards it via a done-callback. This guard catches a
+regression where a copy-paste re-introduces a bare asyncio.create_task.
+
+This is the routes/chat_helpers.py-scoped sibling of the webhook-emitter
+guard added in #4336 (tests/test_webhook_emitters_use_manager.py).
+"""
+import ast
+from pathlib import Path
+
+CHAT_HELPERS = (
+    Path(__file__).resolve().parent.parent / "routes" / "chat_helpers.py"
+)
+
+
+def _untracked_create_task_calls(tree: ast.AST) -> list[tuple[int, str]]:
+    """(lineno, snippet) for any bare asyncio.create_task(...).
+
+    A call is "bare" when its return value is dropped — i.e. it is the direct
+    expression of an ast.Expr statement. Captured forms (`x = asyncio.create_task(...)`,
+    `[asyncio.create_task(...), ...]`, `await asyncio.create_task(...)`) are fine
+    because something else holds the reference.
+
+    The helper itself (_spawn_bg) is exempt: it calls asyncio.create_task once
+    and registers the task in _BG_TASKS before returning.
+    """
+    hits: list[tuple[int, str]] = []
+
+    def _is_create_task(call: ast.Call) -> bool:
+        f = call.func
+        return (
+            isinstance(f, ast.Attribute)
+            and f.attr == "create_task"
+            and isinstance(f.value, ast.Name)
+            and f.value.id == "asyncio"
+        )
+
+    spawn_helper_lines: set[int] = set()
+    for node in ast.walk(tree):
+        if isinstance(node, ast.FunctionDef) and node.name == "_spawn_bg":
+            for n in ast.walk(node):
+                if hasattr(n, "lineno"):
+                    spawn_helper_lines.add(n.lineno)
+
+    for node in ast.walk(tree):
+        if not isinstance(node, ast.Expr):
+            continue
+        if not isinstance(node.value, ast.Call):
+            continue
+        if not _is_create_task(node.value):
+            continue
+        if node.lineno in spawn_helper_lines:
+            continue
+        hits.append((node.lineno, ast.unparse(node.value)))
+    return hits
+
+
+def test_no_untracked_create_task_in_chat_helpers():
+    tree = ast.parse(CHAT_HELPERS.read_text(), filename=str(CHAT_HELPERS))
+    offenders = _untracked_create_task_calls(tree)
+    assert not offenders, (
+        "Background tasks scheduled from routes/chat_helpers.py must go through "
+        "_spawn_bg(coro) so the task is registered in _BG_TASKS and survives until "
+        "it finishes. Found bare asyncio.create_task(...) call(s):\n  "
+        + "\n  ".join(f"chat_helpers.py:{ln}: {snip}" for ln, snip in offenders)
+    )