Convert a value to `Path` when the parameter is annotated `Path`

[vertix]

What to do

Convert a value to Path when the target parameter's annotation asks for Path. The resolver
already reads the signature.

Why

configuronic builds a value from the CLI text with ast.literal_eval, and keeps a str when that
read fails. An int, a float and a bool survive that read. A path does not. So
--output_dir=/tmp/run reaches the function as a str. An .override string reaches it unchanged
for the same result.

A caller cannot annotate the parameter Path today. The annotation disagrees with the value that
arrives, and a type checker believes the annotation. So a path parameter carries str | Path | None,
and the function converts the value again.

The URL case

A path parameter also takes a URL, for example --output_dir=s3://bucket/run. The remote store parses
that URL. Path keeps the text on Linux and on macOS, and changes a separator on Windows. Write a
test for this case.

Who waits for it

positronic annotates each path parameter str | Path | None for this reason. It drops the str half
after this change. prepare_output_dir in positronic/cli/eval/run.py carries a TODO that points
here.