fix: recurse through nested JSON file inputs

[immanuwell]

cog predict --json only rewrites top level @file values rn. If those paths sit inside arrays or nested objects, they go through as raw @... strings, so list[Path] style inputs via JSON are kinda busted.

This makes the rewrite recursive for JSON arrays and objects. Added a unit test and an integration test too.

Repro:
cog build -t $TEST_IMAGE
cog predict $TEST_IMAGE --json '{"paths": ["@1.txt", "@2.txt"]}'

Before this, the CLI sends literal @1.txt strings and the prediction falls over.
After this, those entries become data URLs and it works.

Related: #1344

[markphelps]

you didnt introduce this, and its called out in the docs that @ is reserved for file paths, but Im thinking in the future we should somehow make this more resilient or atleast make the error message better , since this will fail if a user tries to pass in @string thats not a file path. like:

{"metadata":{"handle":"@alice"}} or {"tags":["@foo"]}