Revert `mount_path` parameter from `FastMCP` #4119

Job	Run time
checks / pre-commit	32s
checks / readme-snippets	9s
checks / test (3.10, locked, windows-latest)	2m 14s
checks / test (3.11, lowest-direct, ubuntu-latest)	48s
checks / test (3.12, lowest-direct, ubuntu-latest)	51s
checks / test (3.10, lowest-direct, ubuntu-latest)	48s
checks / test (3.11, locked, windows-latest)	2m 22s
checks / test (3.12, locked, windows-latest)	3m 16s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 11s
checks / test (3.13, lowest-direct, windows-latest)	2m 42s
checks / test (3.12, locked, ubuntu-latest)	54s
checks / test (3.14, lowest-direct, windows-latest)	2m 18s
checks / test (3.10, lowest-direct, windows-latest)	2m 23s
checks / test (3.14, locked, ubuntu-latest)	2m 13s
checks / test (3.11, lowest-direct, windows-latest)	3m 8s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.13, locked, windows-latest)	2m 20s
checks / test (3.13, lowest-direct, ubuntu-latest)	46s
checks / test (3.10, locked, ubuntu-latest)	46s
checks / test (3.14, locked, windows-latest)	3m 11s
checks / test (3.12, lowest-direct, windows-latest)	3m 14s
checks / test (3.11, locked, ubuntu-latest)	45s
	37m 40s

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