Remove unused mcpserver.exceptions.ValidationError #3725

Job	Run time
checks / docs	46s
checks / readme-snippets	11s
checks / pre-commit	50s
checks / test (3.11, lowest-direct, windows-latest)	2m 0s
checks / test (3.10, locked, ubuntu-latest)	1m 30s
checks / test (3.12, lowest-direct, windows-latest)	2m 25s
checks / test (3.11, locked, ubuntu-latest)	1m 28s
checks / test (3.14, lowest-direct, windows-latest)	1m 52s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 24s
checks / test (3.10, locked, windows-latest)	1m 54s
checks / test (3.11, locked, windows-latest)	1m 34s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 27s
checks / test (3.14, locked, ubuntu-latest)	1m 21s
checks / test (3.10, lowest-direct, windows-latest)	1m 52s
checks / test (3.13, lowest-direct, windows-latest)	1m 52s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 20s
checks / test (3.12, locked, ubuntu-latest)	1m 42s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.13, locked, windows-latest)	1m 32s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.12, locked, windows-latest)	1m 43s
checks / test (3.14, locked, windows-latest)	3m 5s
checks / test (3.13, locked, ubuntu-latest)	1m 15s
all-green	4s
	35m 43s

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