fix: correct MCPServer call_tool result type #2820

Job	Run time
checks / pre-commit	40s
checks / readme-snippets	10s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 4s
checks / test (3.10, locked, ubuntu-latest)	59s
checks / test (3.14, lowest-direct, windows-latest)	1m 24s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 6s
checks / test (3.10, locked, windows-latest)	1m 30s
checks / test (3.11, lowest-direct, windows-latest)	1m 29s
checks / test (3.13, lowest-direct, ubuntu-latest)	58s
checks / test (3.11, locked, ubuntu-latest)	59s
checks / test (3.13, locked, windows-latest)	1m 30s
checks / test (3.11, locked, windows-latest)	1m 31s
checks / test (3.13, lowest-direct, windows-latest)	1m 45s
checks / test (3.14, lowest-direct, ubuntu-latest)	58s
checks / test (3.12, lowest-direct, windows-latest)	1m 45s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 3s
checks / test (3.12, locked, windows-latest)	1m 30s
checks / test (3.12, locked, ubuntu-latest)	59s
checks / test (3.10, lowest-direct, windows-latest)	1m 40s
checks / test (3.13, locked, ubuntu-latest)	56s
checks / test (3.14, locked, ubuntu-latest)	2m 1s
checks / test (3.14, locked, windows-latest)	2m 34s
all-green	2s
	28m 33s

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