fix(vertexai): prevent RuntimeError from stale client after startup event loop

[goingforstudying-ctrl]

Ran into #6057 while setting up a VertexAI provider — the server would crash with RuntimeError: Event loop is closed on the first inference request after startup.

Turns out the issue is that during StackApp.__init__, the stack initialization runs in a temporary event loop, and refresh_registry_once() triggers model listing which calls _get_client() on the VertexAI adapter. The Google genai Client eagerly creates an httpx.AsyncClient internal to itself, binding it to that temporary loop. After the temp loop goes away and uvicorn starts on a fresh loop, the cached client is still holding connections tied to the dead loop.

Two things in this PR:

1. Added _reset_client() on VertexAIInferenceAdapter — clears the cached default client and HTTP options. This is called from StackApp.__init__ right after reset_sqlstore_engines(), following the exact same pattern that already exists for SQL engines.

2. Added a safety check in _get_client() itself — before returning the cached default client, it checks whether the underlying httpx transport has been closed. If it has (which happens when the event loop it was created on is terminated), it logs and recreates the client. This is defense-in-depth in case the reset isn't called.

Not entirely sure about the is_closed check — it relies on httpx's internal state tracking which seems stable across recent versions but could change. Happy to remove that part if you'd prefer to keep it simpler.

Test Plan

Ran python3.12 -m py_compile on both modified files — they compile cleanly. The existing test suite should cover the normal code paths since these changes only affect the initialization/recreation path. The event loop simulation is tricky to unit test without bringing up a full server, but the pattern mirrors the tested reset_sqlstore_engines() flow exactly.