Revert `mount_path` parameter from `FastMCP` #4118

Job	Run time
checks / readme-snippets	14s
checks / pre-commit	33s
checks / test (3.10, lowest-direct, windows-latest)	2m 22s
checks / test (3.10, locked, ubuntu-latest)	44s
checks / test (3.10, lowest-direct, ubuntu-latest)	48s
checks / test (3.11, lowest-direct, ubuntu-latest)	46s
checks / test (3.11, lowest-direct, windows-latest)	2m 25s
checks / test (3.10, locked, windows-latest)	2m 15s
checks / test (3.11, locked, ubuntu-latest)	45s
checks / test (3.11, locked, windows-latest)	2m 25s
checks / test (3.12, lowest-direct, ubuntu-latest)	49s
checks / test (3.12, locked, ubuntu-latest)	52s
checks / test (3.12, lowest-direct, windows-latest)	3m 28s
checks / test (3.12, locked, windows-latest)	3m 18s
checks / test (3.13, lowest-direct, ubuntu-latest)	43s
checks / test (3.13, lowest-direct, windows-latest)	2m 21s
checks / test (3.13, locked, ubuntu-latest)	42s
checks / test (3.13, locked, windows-latest)	2m 18s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 5s
checks / test (3.14, lowest-direct, windows-latest)	3m 13s
checks / test (3.14, locked, ubuntu-latest)	2m 13s
checks / test (3.14, locked, windows-latest)	3m 8s
	37m 27s

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