* test(integration): run the Langfuse DB-callback test on its own scratch database
The test from #43282 wrote success_callback=langfuse and the LANGFUSE_* env
into the shared integration LiteLLM_Config. The suite's long-running gateway
reloads that table and only ever adds callbacks, so it kept exporting to the
test's closed Langfuse fake for the rest of the shard even after the rows were
restored. The owned proxy now gets a scratch database, which also removes the
snapshot/restore code. scratch_database moves into _support/database.py so
test_cache_and_quota and this test share one copy, and the stock-config guard
now checks the callback settings instead of the raw YAML text.
* test(integration): include failure_callback in the stock-config Langfuse guard
dd63637322