fix: Json feature flag missing imports

[sunadase]

Fixes: #18258

Some codepaths included in json flag needed imports that are only supplied by possibly other (parquet/ipc + parquet) flags but not with json flag and polars_io::SerReader was missing altogether.

Not sure if this is the way you handle flags for this repo, you can let me know if within function imports are preferred? It could be better in case of use polars_io::SerReader's case, since requiring count_rows_ndjson function is behind a json flag already.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.21%. Comparing base (84d6f80) to head (480bbf1).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18305      +/-   ##
==========================================
- Coverage   80.23%   80.21%   -0.03%     
==========================================
  Files        1501     1501              
  Lines      198943   198915      -28     
  Branches     2837     2837              
==========================================
- Hits       159616   159553      -63     
- Misses      38799    38834      +35     
  Partials      528      528              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ritchie46]

Thanks