fix: enable parallel test execution with pytest-xdist in CI workflow

[deependujha]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

partial work on #612
follow-up work from #608 and #338

OS & Python Version	before	after xdist
ubuntu-22.04: py: 3.09	36m 07s	27m 50s
ubuntu-22.04: py: 3.10	34m 27s	26m 48s
ubuntu-22.04: py: 3.11	35m 05s	27m 16s
ubuntu-22.04: py: 3.12	46m 42s	31m 26s
ubuntu-22.04: py: 3.13	35m 59s	26m 34s
macos-14: py: 3.09	42m 42s	26m 02s
macos-14: py: 3.10	33m 10s	29m 33s
macos-14: py: 3.11	41m 29s	28m 51s
windows-2022: py: 3.09	22m 10s	26m 18s
windows-2022: py: 3.10	23m 00s	24m 03s
windows-2022: py: 3.11	23m 18s	15m 00s
windows-2022: py: 3.12	26m 29s	20m 50s

before time from: https://github.com/Lightning-AI/litData/actions/runs/16801917579/job/47585093769

after time from: https://github.com/Lightning-AI/litData/actions/runs/16803082260/job/47588778262

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84%. Comparing base (5ac0863) to head (f59b9ed).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff          @@
##           main   #620    +/-   ##
====================================
+ Coverage    84%    84%    +1%     
====================================
  Files        52     52            
  Lines      7144   7021   -123     
====================================
- Hits       5968   5902    -66     
+ Misses     1176   1119    -57     

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[tchaton]

Looks great ! Thanks ;)