refactor(rust): Remove duplicated code for expression output order resolution

[nameexhaustion]

The pushdown/pullup stages of expression order resolution currently exist as 2 separate functions with duplicate logic. This is not ideal, as it causes duplication of expression order rules, some of which can be extremely complex.

This PR eliminates one of the codepaths, making it so that we use a single codepath for both.

Detail

The intuition is that you can achieve the effect of is_output_ordered() by simply calling get_observable_orders() (renamed from get_frame_observing) with the output ordering of Column(_) nodes set to O::Independent/O::None instead of O::Frame. This effectively disables the Err(FrameOrderObserved) short-circuiting, ensuring that the result will always be Ok(observable_orders).

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.85%. Comparing base (8d64e3d) to head (9eb6e31).
⚠️ Report is 58 commits behind head on main.

Files with missing lines	Patch %	Lines
...-plan/src/plans/optimizer/set_order/expr_pullup.rs	75.00%	2 Missing ⚠️
...lan/src/plans/optimizer/set_order/expr_pushdown.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24605      +/-   ##
==========================================
+ Coverage   81.84%   81.85%   +0.01%     
==========================================
  Files        1689     1689              
  Lines      230044   230086      +42     
  Branches     2974     2974              
==========================================
+ Hits       188273   188335      +62     
+ Misses      41018    40998      -20     
  Partials      753      753              

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