Return void from McpStatelessSyncServer#closeGracefully instead of Mono #502

Job	Run time
Client Conformance (initialize)	56s
Client Conformance (tools_call)	46s
Server Conformance	57s
Client Conformance (sse-retry)	50s
Client Conformance (elicitation-sep1034-client-defaults)	47s
Auth Conformance (auth/scope-step-up)	51s
Auth Conformance (auth/basic-cimd)	50s
Auth Conformance (auth/token-endpoint-auth-post)	52s
Auth Conformance (auth/scope-from-www-authenticate)	52s
Auth Conformance (auth/metadata-var2)	42s
Auth Conformance (auth/token-endpoint-auth-basic)	50s
Auth Conformance (auth/scope-retry-limit)	48s
Auth Conformance (auth/metadata-default)	51s
Auth Conformance (auth/metadata-var1)	42s
Auth Conformance (auth/pre-registration)	50s
Auth Conformance (auth/scope-omitted-when-undefined)	44s
Auth Conformance (auth/scope-from-scopes-supported)	48s
Auth Conformance (auth/metadata-var3)	43s
Auth Conformance (auth/token-endpoint-auth-none)	50s
	15m 29s

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