FastMCP: Improve structured output for dict and mixed lists #3662

Job	Run time
checks / pre-commit	30s
checks / readme-snippets	14s
checks / test (3.10, lowest-direct, ubuntu-latest)	45s
checks / test (3.10, lowest-direct, windows-latest)	1m 57s
checks / test (3.10, highest, windows-latest)	2m 10s
checks / test (3.10, highest, ubuntu-latest)	41s
checks / test (3.11, highest, windows-latest)	2m 34s
checks / test (3.11, lowest-direct, ubuntu-latest)	42s
checks / test (3.12, lowest-direct, windows-latest)	3m 19s
checks / test (3.11, highest, ubuntu-latest)	45s
checks / test (3.12, highest, windows-latest)	3m 49s
checks / test (3.12, lowest-direct, ubuntu-latest)	46s
checks / test (3.13, lowest-direct, windows-latest)	2m 3s
checks / test (3.11, lowest-direct, windows-latest)	1m 59s
checks / test (3.12, highest, ubuntu-latest)	48s
checks / test (3.13, lowest-direct, ubuntu-latest)	41s
checks / test (3.13, highest, windows-latest)	2m 6s
checks / test (3.13, highest, ubuntu-latest)	44s
	26m 33s

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