[Bug] stability_digest_payload_from_ce_payload fails with AttributeError: 'dict' object has no attribute 'split'

**Bug Description**
When using @crashlytics_fn.on_stability_digest_published(), the function fails with an AttributeError because digestDate is received as a dictionary (Firebase Timestamp object) instead of a string, but the SDK attempts to call .split() on it.

**Error Message**
AttributeError: 'dict' object has no attribute 'split'

**Full Stack Trace**
Exception on / [POST]
Traceback (most recent call last):
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/flask/app.py", line 1511, in wsgi_app
response = self.full_dispatch_request()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/flask/app.py", line 919, in full_dispatch_request
rv = self.handle_user_exception(e)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/flask/app.py", line 917, in full_dispatch_request
rv = self.dispatch_request()
^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/flask/app.py", line 902, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args) # type: ignore[no-any-return]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/functions_framework/execution_id.py", line 157, in wrapper
result = view_function(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/functions_framework/__init__.py", line 188, in view_func
function(event)
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/firebase_functions/alerts/crashlytics_fn.py", line 293, in crashlytics_decorator_wrapped
func(crashlytics_event_from_ce(raw))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/firebase_functions/private/_alerts_fn.py", line 243, in crashlytics_event_from_ce
return event_from_ce_helper(raw, CrashlyticsEvent)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/firebase_functions/private/_alerts_fn.py", line 209, in event_from_ce_helper
"data": firebase_alert_data_from_ce(event_dict),
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/firebase_functions/private/_alerts_fn.py", line 175, in firebase_alert_data_from_ce
alert_payload = stability_digest_payload_from_ce_payload(alert_payload)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/firebase_functions/private/_alerts_fn.py", line 136, in stability_digest_payload_from_ce_payload
digest_date=_util.timestamp_conversion(payload["digestDate"]),
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/firebase_functions/private/util.py", line 407, in timestamp_conversion
precision_timestamp = get_precision_timestamp(time)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.11/site-packages/firebase_functions/private/util.py", line 391, in get_precision_timestamp
_, s_fraction = time.split(".")
^^^^^^^^^^
AttributeError: 'dict' object has no attribute 'split'

**Environment**
firebase-functions version: 0.5.0 (also happens in 0.4.x)
firebase-admin version:7.1.0
Python version: 3.11
Deployment environment: Google Cloud Functions (2nd gen)

Commencez par stability_digest_payload_from_ce_payload dans firebase_functions/private/_alerts_fn.py et suivez son appel à timestamp_conversion dans firebase_functions/private/util.py. Reproduisez l’événement de stability digest en utilisant le digestDate sous forme de dictionnaire présenté dans le rapport, puis vérifiez que la conversion de l’événement s’effectue sans l’AttributeError et produit l’horodatage attendu.

Rédigé par le modèle d'indexation à partir du texte de l'issue.