fix(webhooks): route public emitters through fire_and_forget (#3964) (#4336)

The three public webhook emitters in chat_helpers and webhook_routes
schedule deliveries via asyncio.create_task(webhook_manager.fire(...)),
which bypasses WebhookManager._bg_tasks. asyncio only holds a weak
reference to the outer task, so the GC can collect it mid-delivery and
the webhook is silently dropped.

Route all three through webhook_manager.fire_and_forget() so the task
is tracked by _spawn_tracked() and the manager owns the full lifecycle.

Adds an AST-level guard test that scans routes/ for direct
asyncio.create_task wrapping webhook_manager.fire(...) to prevent
regressions.

tests/test_webhook_emitters_use_manager.py

@@ -0,0 +1,54 @@
+"""Guard: every public webhook emitter goes through the manager.
+
+Public emitters in `routes/` must schedule their fire through
+`webhook_manager.fire_and_forget(...)` (or `_spawn_tracked`). A bare
+`asyncio.create_task(webhook_manager.fire(...))` escapes
+`WebhookManager._bg_tasks`, so asyncio only holds a weak reference to the
+delivery task and the GC can collect it before it sends — silently dropping
+the webhook. Catching this with a scan stops a regression from sneaking
+back in via a copy-paste.
+"""
+import ast
+from pathlib import Path
+
+ROUTES_DIR = Path(__file__).resolve().parent.parent / "routes"
+
+
+def _untracked_fire_calls(tree: ast.AST) -> list[tuple[int, str]]:
+    """Return (lineno, snippet) for any asyncio.create_task(webhook_manager.fire(...))."""
+    hits: list[tuple[int, str]] = []
+    for node in ast.walk(tree):
+        if not isinstance(node, ast.Call):
+            continue
+        func = node.func
+        if not (isinstance(func, ast.Attribute) and func.attr == "create_task"):
+            continue
+        if not (isinstance(func.value, ast.Name) and func.value.id == "asyncio"):
+            continue
+        if not node.args:
+            continue
+        inner = node.args[0]
+        if not isinstance(inner, ast.Call):
+            continue
+        inner_func = inner.func
+        if (
+            isinstance(inner_func, ast.Attribute)
+            and inner_func.attr == "fire"
+            and isinstance(inner_func.value, ast.Name)
+            and inner_func.value.id == "webhook_manager"
+        ):
+            hits.append((node.lineno, ast.unparse(node)))
+    return hits
+
+
+def test_no_untracked_webhook_fire_in_routes():
+    offenders: list[str] = []
+    for path in ROUTES_DIR.rglob("*.py"):
+        tree = ast.parse(path.read_text(), filename=str(path))
+        for lineno, snippet in _untracked_fire_calls(tree):
+            offenders.append(f"{path.relative_to(ROUTES_DIR.parent)}:{lineno}: {snippet}")
+    assert not offenders, (
+        "Public webhook emitters must use webhook_manager.fire_and_forget(...) "
+        "so the delivery task is tracked in WebhookManager._bg_tasks. Found "
+        "untracked emitter(s):\n  " + "\n  ".join(offenders)
+    )