fix: return resource-not-found code from MCPServer #2564

Job	Run time
checks / readme-snippets	14s
checks / pre-commit	32s
checks / test (3.10, lowest-direct, ubuntu-latest)	50s
checks / test (3.11, lowest-direct, windows-latest)	1m 18s
checks / test (3.10, locked, windows-latest)	1m 27s
checks / test (3.12, locked, ubuntu-latest)	50s
checks / test (3.12, locked, windows-latest)	1m 8s
checks / test (3.10, lowest-direct, windows-latest)	1m 18s
checks / test (3.11, locked, ubuntu-latest)	49s
checks / test (3.12, lowest-direct, ubuntu-latest)	39s
checks / test (3.11, lowest-direct, ubuntu-latest)	43s
checks / test (3.14, locked, ubuntu-latest)	1m 54s
checks / test (3.10, locked, ubuntu-latest)	50s
checks / test (3.14, lowest-direct, ubuntu-latest)	49s
checks / test (3.12, lowest-direct, windows-latest)	1m 19s
checks / test (3.11, locked, windows-latest)	1m 24s
checks / test (3.13, locked, windows-latest)	1m 12s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.14, lowest-direct, windows-latest)	1m 13s
checks / test (3.13, lowest-direct, windows-latest)	1m 26s
checks / test (3.14, locked, windows-latest)	2m 7s
checks / test (3.13, lowest-direct, ubuntu-latest)	49s
all-green	4s
	23m 43s

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