fix(core): rename parameter f to _f in _handle to prevent keyword argument collisions

[Hasnaathussain]

When route handlers accept an argument named f (representing a file, form, or general parameter), invoking the handler via FastHTML's _handle function raises TypeError: _handle() got multiple values for argument 'f'.

This is because the first positional argument of _handle is named f, which collides with the keyword argument unpacked from the request payload.

This PR renames the parameter in _handle from f to _f (both in the source notebook nbs/api/00_core.ipynb and the exported fasthtml/core.py) to prevent keyword collisions.

I've also added a regression test demonstrating route registration and invocation with parameter f working seamlessly.

Closes #834

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[jph00]

In nbdev we don't put tests in a separate file. Please follow the project's coding style.