Add ModelContextProtocol.Core package #392

Job	Run time
build (ubuntu-latest, Debug)	2m 29s
build (macos-latest, Debug)	1m 40s
build (windows-latest, Debug)	3m 6s
build (ubuntu-latest, Release)	3m 21s
build (windows-latest, Release)	3m 10s
build (macos-latest, Release)	1m 57s
publish-coverage / publish-coverage	39s
	16m 22s

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