fix(shared): preserve MCPError payload on pickle #1965

Job	Run time
checks / pre-commit	33s
checks / readme-snippets	12s
checks / test (3.11, lowest-direct, ubuntu-latest)	58s
checks / test (3.10, lowest-direct, windows-latest)	2m 31s
checks / test (3.10, locked, ubuntu-latest)	56s
checks / test (3.10, lowest-direct, ubuntu-latest)	57s
checks / test (3.12, locked, windows-latest)	2m 54s
checks / test (3.12, lowest-direct, ubuntu-latest)	58s
checks / test (3.13, lowest-direct, windows-latest)	2m 31s
checks / test (3.12, lowest-direct, windows-latest)	3m 43s
checks / test (3.13, locked, windows-latest)	2m 53s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 14s
checks / test (3.12, locked, ubuntu-latest)	1m 6s
checks / test (3.14, locked, ubuntu-latest)	2m 23s
checks / test (3.14, locked, windows-latest)	3m 16s
checks / test (3.11, lowest-direct, windows-latest)	2m 42s
checks / test (3.11, locked, windows-latest)	2m 52s
checks / test (3.11, locked, ubuntu-latest)	54s
checks / test (3.13, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, locked, windows-latest)	2m 42s
checks / test (3.14, lowest-direct, windows-latest)	2m 33s
checks / test (3.13, locked, ubuntu-latest)	49s
all-green	4s
	40m 38s

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