Remove deprecated mount/import argument order (deprecated 2.8.0+)

[jlowin]

Remove support for the deprecated argument order in mount() and import_server() methods.

Old order: mount("prefix", server) / import_server("prefix", server)
New order: mount(server, "prefix") / import_server(server, "prefix")

Also removes support for deprecated separator parameters:

• tool_separator
• resource_separator
• prompt_separator

Deprecated in: 2.8.0+

Files affected:

• src/fastmcp/server/server.py