chore(ci): add Python 3.14 to the testing matrix

[bhimrazy]

What does this PR do?

This PR adds support for python 3.14 in testing matrix.

• Updates the CI matrix to test these versions, excludes Windows for 3.14, and removes old macOS exclusions.
• Updates setup classifiers
• Fixes a dataloader worker index pickle issue causing in 3.14. ref: https://github.com/Lightning-AI/litData/actions/runs/19125632201/job/54655211247.

  TypeError: cannot pickle 'itertools.cycle' object
  when serializing dict item '_worker_idx'
  when serializing litdata.streaming.dataloader.StreamingDataLoader state
  when serializing litdata.streaming.dataloader.StreamingDataLoader object
  ...

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80%. Comparing base (8437a03) to head (ffa6d00).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #747   +/-   ##
===================================
  Coverage    80%    80%           
===================================
  Files        52     52           
  Lines      7331   7333    +2     
===================================
+ Hits       5873   5875    +2     
  Misses     1458   1458           

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