Make CallToolResult.IsError optional (#573) #520

Job	Run time
build (ubuntu-latest, Debug)	3m 37s
build (ubuntu-latest, Release)	3m 19s
build (windows-latest, Release)	5m 16s
build (windows-latest, Debug)	4m 51s
build (macos-latest, Release)	1m 34s
build (macos-latest, Debug)	3m 6s
publish-coverage	0s
	21m 43s

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