_finalize_all_traces called _get_langfuse(), which lazily BUILDS a client when the
launch-profile slot is empty. Under a multiplex gateway every turn runs scoped, so
only the per-home slots are populated; at exit (no scope) the credential read
raised UnscopedSecretError and the flush loop was skipped for every profile,
losing all pending traces. The finalizer now iterates the already-settled
clients (launch slot + per-home map) and never initializes. Test drives two
profiles under real home-override + secret scopes, then finalizes unscoped.
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