Non-secret caller key used for request cleanup/rate scopes.
Credential resolution suitable for HTTP discovery-mode server creation.
Directory scanners and MCP inspectors need to initialize and enumerate tools
before a user supplies real B2 credentials. The placeholder credential is
never sent to B2: callers must pass the returned config to createServer
with credentialsUnavailable: true, which short-circuits all tool execution.
Build a non-secret placeholder credential resolution for HTTP MCP discovery.