feat: Add maintain_order parameter to merge_sorted

[jonathanchang31]

Summary

Closes #27114.

Adds a maintain_order: bool parameter to merge_sorted(). When set to True, the output is guaranteed to have left-biased ordering for equal keys: rows from the left frame always appear before rows from the right frame when their keys match.

• Threads maintain_order through the full stack: Python API → PyO3 bindings → DSL/IR plan → streaming engine → in-memory engine
• Core streaming fix: find_mergeable() holds back right-side rows at chunk boundaries whose keys equal the left's maximum (uses gt_eq instead of gt), preventing right-side ties from being emitted before left-side ties arriving in later morsels
• The in-memory engine already produces left-biased output, so the flag is only load-bearing in the streaming path
• Defaults to False for backward compatibility

Test plan

• 3 new test functions (6 cases) covering both streaming and in-memory engines:
  • Basic left-biased ordering with overlapping keys
  • All keys identical
  • DataFrame.merge_sorted() API path
• All 41 existing merge_sorted tests pass with no regressions
• Rust cargo check --features merge_sorted passes

[codecov]

Codecov Report

❌ Patch coverage is 87.87879% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.59%. Comparing base (880651f) to head (ef3dafe).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/plans/visitor/hash.rs	0.00%	2 Missing ⚠️
...rates/polars-python/src/lazyframe/visitor/nodes.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #27263   +/-   ##
=======================================
  Coverage   81.58%   81.59%           
=======================================
  Files        1820     1820           
  Lines      251036   251083   +47     
  Branches     3149     3149           
=======================================
+ Hits       204808   204867   +59     
+ Misses      45420    45408   -12     
  Partials      808      808           

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