test(client-runtime): each Read key signs its own asset URL

Proves through the production asset query family that one key shares a
request, a new key signs afresh, and the key never reaches the remote or
local media fallback RPC.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
423865defbJulius Marminge committed on 10/22/2026, 8:35:20 AMparent83ba08b
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