cached_sdk_client returned the client cached on tools.web_tools before it
read the key, so the Exa, Parallel and AsyncParallel clients kept the key
they were first built with for the life of the process. A key fixed in .env
and applied with /reload still sent the old key (401s until a restart), and
on a gateway serving multiplexed profiles every profile's Exa and Parallel
calls went out on whichever profile's key built the client first, billed to
that account. A key removed from the environment also kept being used.
Resolve the key on every call and reuse the cached client only when it was
built with that key; the slot now holds (key, client) as one value so two
builds racing under different keys cannot record one key beside the other
key's client. Firecrawl already compares its credential before reusing its
client; this brings the two SDK-backed providers in line.
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