Without an explicit provider the probe runs the startup pin and then
the configured fallback chain. When the chain only resolves at its tail
(e.g. BWS-backed secrets not yet hydrated during backend boot), the
resolved runtime stops on the tail provider and the failure surfaced
as 'No usable credentials found for <tail>' — blaming a provider the
user never pinned and sending the post-update onboarding overlay's
reason box after the wrong provider (#124939).
Attribute the failure to the pin instead: the startup provider pin,
else the config model provider — the same primary notion
pre_agent_fallback_notice already uses. Auto mode (no pin anywhere)
keeps naming the resolved route's provider.
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