FastMCP: Improve structured output for dict and mixed lists #3663

Job	Run time
checks / pre-commit	31s
checks / readme-snippets	14s
checks / test (3.10, lowest-direct, windows-latest)	2m 11s
checks / test (3.10, lowest-direct, ubuntu-latest)	42s
checks / test (3.10, highest, ubuntu-latest)	43s
checks / test (3.10, highest, windows-latest)	2m 0s
checks / test (3.11, highest, windows-latest)	2m 12s
checks / test (3.11, lowest-direct, windows-latest)	2m 0s
checks / test (3.11, lowest-direct, ubuntu-latest)	42s
checks / test (3.13, lowest-direct, ubuntu-latest)	43s
checks / test (3.12, highest, windows-latest)	2m 54s
checks / test (3.11, highest, ubuntu-latest)	45s
checks / test (3.12, lowest-direct, windows-latest)	3m 20s
checks / test (3.12, lowest-direct, ubuntu-latest)	46s
checks / test (3.12, highest, ubuntu-latest)	47s
checks / test (3.13, highest, ubuntu-latest)	46s
checks / test (3.13, lowest-direct, windows-latest)	2m 30s
checks / test (3.13, highest, windows-latest)	2m 38s
	26m 24s

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