fix: tighten MCPServer tool result typing #2729

Job	Run time
checks / pre-commit	29s
checks / readme-snippets	11s
checks / test (3.11, lowest-direct, windows-latest)	1m 21s
checks / test (3.11, lowest-direct, ubuntu-latest)	59s
checks / test (3.10, locked, windows-latest)	1m 27s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.10, lowest-direct, windows-latest)	1m 16s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.13, lowest-direct, ubuntu-latest)	52s
checks / test (3.11, locked, ubuntu-latest)	54s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.11, locked, windows-latest)	1m 25s
checks / test (3.14, locked, windows-latest)	2m 19s
checks / test (3.12, lowest-direct, ubuntu-latest)	56s
checks / test (3.13, lowest-direct, windows-latest)	1m 44s
checks / test (3.12, lowest-direct, windows-latest)	1m 32s
checks / test (3.12, locked, windows-latest)	1m 26s
checks / test (3.13, locked, windows-latest)	1m 17s
checks / test (3.14, lowest-direct, ubuntu-latest)	55s
checks / test (3.14, locked, ubuntu-latest)	1m 57s
checks / test (3.14, lowest-direct, windows-latest)	1m 11s
checks / test (3.13, locked, ubuntu-latest)	50s
all-green	3s
	25m 49s

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