fix: report correct number of items left in the iterator for IncrementalSortIterator

[rluvaton]

Which issue does this PR close?

N/A

Rationale for this change

size_hint for IncrementalSortIterator returns the initial number of items rather than how many items left in the iterator

What changes are included in this PR?

update size hint to consider the number of elements already produced, impl ExactSizeIterator and added test

What is the testing strategy for this PR?

unit test

Are there any user-facing changes?

no

[EmilyMatt]

LGTM, my bad 😐

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.66667% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.80%. Comparing base (a5c809f) to head (5e96426).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/sorts/stream.rs	86.66%	0 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25107      +/-   ##
==========================================
+ Coverage   81.74%   81.80%   +0.05%     
==========================================
  Files        1128     1130       +2     
  Lines      416644   417793    +1149     
  Branches   416644   417793    +1149     
==========================================
+ Hits       340592   341777    +1185     
+ Misses      55995    55882     -113     
- Partials    20057    20134      +77     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.