A promise that resolves after the stdio transport has been registered with the MCP SDK.
The stdio path reads credentials from the process environment, attempts
capability discovery once with a B2_STDIO_CAPABILITY_TIMEOUT_MS bootstrap
deadline (10s by default). A local deadline expiry starts with an empty
fail-closed capability set; a returned transient upstream outage degrades to
the full tool surface. When no application key is present, or B2 definitively
rejects the supplied key during this bootstrap lookup, stdio starts a
credential-less discovery server instead of exiting.
Start the MCP server over stdio.