Two problems on the credentialFallback path, where a search provider
borrows a chat provider's connection:
- the lock was attributed to the search provider id, but the connection
belongs to the chat provider, so markAccountUnavailable looked it up
under the wrong provider and read a stale backoffLevel
- with no model argument the lock key is `modelLock___all`, which
isModelLockActive treats as blocking every model — one failing search
would have taken the shared glm key offline for chat as well
Attribute the lock to the provider that owns the connection, and scope
it to `websearch:<provider>`, passed to getProviderCredentials too so
the lock is read back under the same key.
ec6692808b