tools/call answered with one JSON object, so a handler's progress and log
notifications had nowhere to go — and reached for a fiber that was not
there, coming back as -32603. Handlers now run in a fiber and the stream
opens after the first suspension, late enough that an error still gets the
status the spec fixes for it. logLevel finally gates the log messages it
was already being parsed for, and a server-initiated request is refused by
name rather than as a FiberError.
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